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HUMAN PHYSIOLOGY; 



DESIGNED FOB T H I 



USE OF THE HIGHER CLASSES IN COMMON SCHOOLS. 



By GEORGE HAYWARD, 31. D. 



i 




1871 X^> 



BOSTON : 

MARSH, CAPEN & LYON, 

18 34. 



Entered according to Act of Congress in the year 1834, by 

George Hayward, 

in the Clerk's Office of llie District Court of the District 

of Massachusetts. 



^ '7^ 



BOSTON; 
3s B. Dow, 
1-2 Washington St. 



' James B. Dow, Printer, > 



PREFACE. 



This work is intended for those who are unac- 
v/jainted with the structure of the human body. It is 
an attempt to explain to them the uses of its most im- 
portant parts, in familiar and popular language. It dif- 
fers, therefore, in this respect, from most works on 
physiology, which suppose some knowledge of anato- 
my in those who read them ; and it does not treat of 
those topics, which, though highly important to profes- 
sional students, could not with propriety be introduced 
into the studies of the young. 

It was thought that a knowledge of the functions of 
the principal organs of our bodies, would be not only 
useful, but interesting ; and that the study of human 
physiology might prove as agreeable as that of Botany 
or Mineralogy. It is difficult, no doubt, to p:*esent 
the subject in a form, at once popular and intelligible : 
it is not easy to avoid the use of technical phrases, and 
in some cases it is impossible to find substitutes for 
them. In every instance in which they have been em- 
ployed in this work, however, they have been explain- 
ed, and it is hoped in a manner so simple, as to render 



PREFACE. 



them intelligible to every reader. It was of course a 
primary object to make it easy of comprehension ; and 
when it is recollected, that it was not WTitten for pro- 
fessional students, it is thought that no apology will be 
deemed necessary for the explanation that has been 
given on many points, which to them may not seem to 
require it. 

It is hardly necessary to add, that this little w^ork 
makes no claims to originality. It has been compiled 
with some degree of care from the best authorities on 
the subject, and the materials, which the labor and re- 
search of others have collected, have been freely em- 
ployed. The compiler fears that it is by no means 
free from errors ; he is aware that he could not bring to. 
the task all the knowledge of the subject that was de- 
sirable, and he executed it while engaged to some ex- 
tent in the active duties of a responsible profession. 
He was induced to undertake it, in the hope that it 
might be useful to the young ; and his object will be 
effected, if it should open to them a new and interest- 
esting branch of knowledge. He was desirous that 
they should become acquainted with human physiology, 
as he felt confident, that they could not fail to see in 
the structure and functions of their own bodies, the 
clearest evidence of wonderful contrivance and benefi- 
cent wisdom. 

Boston^ March, 1834. 



CONTENTS. 



Introduction 13 

CHAPTER L 
Of Digestion 27 

CHAPTER II. 
Of the Circulation of the Blood 45 

CHAPTER III. 
Of Resph'ation 6G 

CHAPTER IV. 
Of Animal Heat . . .84 

CHAPTER V. 
Of Secretion 93 

CHAPTER VI. 
Of Absorption 109 

CHAPTER VII. 
Of Nutrition 123 

CHAPTER VIH. 
Of the Nervous System , 133 

CHAPTER IX. 
Of the Sense of Smell 151 



CONTENTS. 

CHAPTER X. 
Of the Sense of Taste 158 

CHAPTER XI. 
Of the Sense of Touch 163 

CHAPTER XII. 
Of the Sense of Heanng 170 

CHAPTER XIII. 
Of the Sense of Sight 181 

CHAPTER XIV. 
Of the Voice 194 

CHAPTER XV. 
Of Locomotion 206 

CHAPTER XVI. 
Of the Decay and Dissolution of the Body 214 



INTRODUCTION, 



Physiology, in its limited sense, is that branch of 
knowledge, which explains the uses of the various or- 
gans of living beings. It is divided into Vegetable and 
Animal ; and the latter is again divided into Compara- 
tive and Human. Comparative Physiology treats of 
the corporeal functions of the inferior animals ; while 
Human Physiology of course explains those of man. 

Physiologists at the present day, do not, as was for- 
merly done, refer the actions of the living body to any 
one single vital principle, nor do they attempt to define 
life. It is known only by its properties. The most 
important of these are contractility and sensibility, 
and on these two, the others seem in great measure to 
depend. Contractility belongs to the muscles or the 
agents of motion ; sensibility to the nerves, or the 
agents of sensation. 

The vital properties not only support and nourish 
the body, but enable it to resist all those agents which 
tend to its destruction. The moment life has ceased, 
the body is subjected to the influence of mechanical 
and chemical laws. The blood gravitates in the ves- 
sels, and decomposition ensues with greater or less ra- 
pidity, according to the degree of heat and moisture to 
2 



14 INTRODUCTION. 

which it is exposed. Its temperature, too, becomes 
the same as that of the surrounding atmosphere, though 
before death it was uniformly higher. 

A physiological arrangement has been proposed, 
which, though perhaps not perfectly true in the extent 
to which it has been carried, is by no means destitute 
of convenience. It considers the human system as 
made up of two parts, in some measure distinct, and 
yet united in some points. One of these relates mere- 
ly to the growth or organization of the body ; the other 
to those peculiarities, which distinguish the animal from 
the vegetable. The first has been called organic life, 
and the other, animal life. The functions of the or- 
ganic life are those which tend to the preservation and 
growth of the individual, while those of the animal Hfe 
connect him with external objects. The organs that di- 
gest the food, and those that circulate the blood, be- 
long to the organic life, and those of the voice, the 
senses, and locomotion, form a part of the animal life. 
The organs of the first are for the most part irregular in 
their shape, and are not usually found in pairs. This 
is easily seen by examining the stomach and intestines, 
or the heart and large vessels. 

The organs of animal life, on the contrary, are in 
pairs, and almost always symmetrical, those of one side 
of the body resembling very nearly those of the other ; 
and on this symmetry in some measure depends the 
correctness of their functions. If one eye be injured, 
the other sees less perfectly ; and the same is true of 
all the organs of animal life. 



INTRODUCTION. l5 

This division of the organs, into those of organic and 
those of animal hfe, though highly valuable for many 
purposes, is not strictly accurate in all its parts. There 
are, for example, some of the organs of organic Hfe that 
are in pairs, and that are also perfectly symmetrical, 
while the symmetry is wanting in some of those of the 
animal life. The glands that secrete the saliva, belong 
to the first class ; yet one is found on each side of the 
body exactly resembling the other. The two hemis- 
pheres of the brain, which is one of the principal or- 
gans of animal life, do not, on the contrary, always re- 
semble each other in the size, form and number of their 
convolutions. 

The animal body is composed of fluids and solids. 
The former constitute a much greater part of the whole 
than the latter ; being in the proportion of six to one, 
according to Richerand, and of nine to one, according 
to Chaussier, in an adult subject. Various divisions 
have been made of the human fluids, and a very com- 
mon one is that which is founded on their chemical 
composition. To this it has been justly objected, that 
the constituent elements of all the fluids are not yet 
known, and that of course an arrangement founded on 
their composition, cannot be perfect. The most sim- 
ple arrangement is that which divides them into four 
classes, viz. : 1st, into those which form the blood; 
2d, the blood itself; 3d, those that are separated from 
the blood ; and 4th, those that are returned to it from 
the various parts of the body. All of these will be 
spoken of hereafter ; the first, when treating of diges- 
tion; the second, when speaking of the heart and the 



16 INTRODUCTION. 

circulation ; the third, under the head of secretion ; and 
the fourth, under that of absorption. 

The sohds are formed from the fluids by a process 
called secretion, which will be treated of in a subse- 
quent part of this work. Much labor and research 
have been bestowed on attempts to ascertain how ma- 
ny elementary solids exist in the various organs of the 
body. Some have supposed that all the solids are 
formed from three elementary ones, viz. : the cellular, 
the nervous, and the muscular ; and to these others 
have added the osseous and membranous. It is behev- 
ed that all the organs, however various they may ap-* 
pear, in their structure and composition, can be ulti-» 
mately reduced to these five. 

These simple textures or tissues, as they are called, 
being united in various proportions, form the compound 
tissues or systems. There are eleven of these systems, 
according to the arrangement of Dupuytren and Rich' 
erand : these are 

1. The Cellular. 

2. The Mucous. 

3. The Serous. 

4. The Muscular. 

5. The Osseous. 

6. The Vascular. 

7. The Nervous. 

8. The Fibrous. 

9. The Erectile. 

10. The Horny, as the Nails and Hair. 

11. The Parenchymatous, as the Glands. 



INTRODUCTION. 17 

It may be proper to remark, that these simple sys- 
tems, by combining with each other, form what may be 
called the compound systems, as the circulating, ner- 
vous, and respiratory systems. A single example will 
perhaps make this more clear. The circulating system 
is composed of several parts or organs, and each of 
these is formed by a union of two or more of the sim- 
ple systems. Thus we find in the circulating system, 
the cellular, serous, muscular, vascular, nervous, and 
fibrous systems, if no others. 

Some of these simple systems will now be briefly 
described. 



Of the Cellular System. 

The cellular texture or membrane, extends through- 
out all parts of the body. It not only surrounds every 
organ, but separates the different portions of it. It 
takes its name from its structure, being composed of 
cells, which for the most part communicate with each 
other. It varies in density according to its situation: 
it is thick and firm in parts that are exposed, as under 
the skin on the palms of the hands and the soles of the 
feet ; and it is of extreme dehcacy in other situations, 
as in the brain and in the eye. 

All the fat of the body is deposited in the cells of the 
cellular texture, and is hence called, by anatomists, cel- 
lular substance. The cells which contain the fat, it 
has been said by Dr. Wm. Hunter, do not communi- 
cate with each other, and of course are not the same 
2# 



18 INTRODUCTION. 

as those in which water and air are sometimes ef- 
fused, and which communicate freely. 

There can be no clearer proof of this free communi- 
cation between the cells, than what is furnished by that 
singular affection called emphysema. Sometimes in a 
fracture of a rib, a portion of the bone will wound the 
lungs so as to form a communication between an air 
cell and the cellular texture. The air will then pass in 
respiration into this texture, and gradually extend itself 
all over the body. In some instances suffocation has 
been the consequence of the enormous swelling pro- 
duced in this way. 

The cellular texture forms a great portion of the 
body, every organ being largely supplied with it. Its 
chemical composition is supposed to be gelatinous, or 
jelly-like. 



Of the Mucous System- 

The mucous membrane lines all those cavities which 
communicate with the atmosphere, as the mouth, wind- 
pipe, stomach, and intestines. It takes its name from 
the fluid with which it is constantly covered in health. 
This fluid is secreted by numerous small glands, which 
are situated in the substance of the membrane. The 
mucous membrane is of a reddish color, highly vascu- 
lar, and abundantly supplied in some parts with nervous 
filaments. It has important functions to perform : it is 
the seat of the sense of taste and smell, and contributes 
largely to the process of digestion. 

The surface of the mucous membrane is not smooth; 



INTRODUCTION. 19 

on the contrary, it is covered with small folds or wrin- 
kles, which become much larger under some circum- 
stances, as when subjected to the action of acrid and 
irritating substances. Though this membrane lines in- 
ternal organs, it is always in contact with foreign sub- 
stances, as the food which is taken for nourishment, and 
the air for the purposes of respiration. It is no doubt 
for the purpose of protecting the system from the inju- 
rious effects of these substances, in case any of a dele- 
terious character should be introduced, that this mem- 
brane is ahvays covered with the viscid fluid, from 
which it takes its name. 

Many have supposed that there is a great resemblance 
between the skin and the mucous membrane, and that 
the difference between them is rather owing to their 
situation than to any difference in their intimate struc- 
ture. The parts of this membrane which are the near- 
est the exterior of the body, as the mucous membrane 
of the lips, are certainly very similar to the skin, and 
Bichat thought that if from any accident a portion of 
the mucous membrane should be brought upon the exte- 
rior of the body, and be kept constantly there, it would 
differ but little from the skin itself. " But this conjec- 
ture is not confirmed by fact. In some cases of mal- 
formation, in which a portion of the mucous coat of 
the bladder was situated on the exterior of the body, it 
was found that it retained all the peculiar characters of 
the mucous membranes. 



20 INTRODUCTION, 



Of the Serous System. 

The serous membrane covers on the exterior nearly 
all those organs which are lined with the mucous mem- 
branes, such as the stomach, lungs and intestines. It 
is a colorless, smooth, and delicate membrane, though 
possessing considerable strength and elasticity. It con- 
tains but few blood-vessels, and still fewer nerves, and 
consequently possesses but a small degree of sensibility 
in a state of health. A considerable quantity of serous 
fluid, from which circumstance it takes its name, is 
constantly throw^n out on its surface ; and this is done 
not by glands, as in the mucous membrane, but by the 
blood-vessels themselves, by a process called exhala- 
tion. The membrane itself having a highly pohshed 
surface, and being always lubricated by this fluid, read- 
ily answers the end for which it seems to be designed, 
that is, to enable the organs which it covers to move 
with ease upon each other. 

A striking difference appears in the mucous and se- 
rous membranes when attacked by inflammation. This 
disease produces in the first of these membranes, if it 
be not severe, an increase in the secretion of its ordi- 
nary fluid, or a change in the character of the fluid ; 
but if the disease be serious, the membrane ulcerates 
in spots where the inflammation is greatest. 

When the same disease attacks the serous mem- 
branes, an efflision of adhesive matter takes place on 
the surface, which unites it to the opposite surface. 
This is very frequently seen in inflammation of the 



INTRODUCTION. 21 

membrane covering the lungs, called pleurisy: a close 
adhesion is often formed between this serous membrane 
and the serous membrane which covers the inside of 
the ribs. The fact that the same disease produces such 
different effects in the two membranes, is a remarkable 
proof of design in that superintending wigdom which, 
presides over the animal economy. 

If the mucous membranes were to adhere from the 
same degree of inflammation which produces this ef- 
fect in the serous, but few individuals would attain any 
considerable age. A slight cold would produce death 
by closing the passage to the stomach or the lungs. 

The object of the adhesion in the serous membranes 
when inflamed, is also apparent. In health, they are in 
constant motion, and subjected to a great degree of 
friction, from which no inconvenience is experienced, 
as they are insensible, and constantly moistened by a 
serous fluid, But in disease, they become exquisitely 
tender, and this adhesion is formed to prevent therri 
from moving on each other, 



Of the Muscular System, 

The muscles are that part of the body, which is fa- 
miliarly known by the name of flesh. They are of 
various forms and sizes, according to their situations 
and the purposes they are designed to accomplish. 
They are made up of fibres, each of which is covered 
with a delicate layer of cellular membrane, and the 
whole muscle is again covered by a sheath. They are 
of a red color, which is usually attributed to the nu- 



22 INTRODUCTION. 

merous blood-vessels with which they are supplied. 
This explanation was called in question by Bichat, who 
maintained that the color was owing to some undis- 
covered property inherent in the muscle. He founded 
this opinion on the fact that some muscles are paler 
than others in the same animal, though supphed with 
an equal quantity of blood. His opinion, however, 
has not been generally received, and the difference of 
color, of which he speaks, has been attributed to the 
different degrees of exercise which the muscles receive, 
those being reddest which are most in action. 

The muscles are the agents of locomotion, and for 
this purpose they are attached to the bones, which are 
used as levers, and the body is moved by the contrac- 
tion of the muscles. They are also the agents of all 
the other motions of the body, and they derive their 
power from the nerves with which they are supplied. 
Haller states that the nerves which go to the thumb are 
larger than those sent to the liver. The action of most 
of the muscles, is under the control of the will ; but 
there is an extensive class of muscular membranes, 
whose functions are essential to life, the action of which 
is wholly involuntary. They surround the stomach and 
intestines, form in fact one of their coats, and give them 
that power of motion which is necessary to the per- 
formance of their functions. 

Many attempts have been made to ascertain the form 
of the ultimate elements of the muscles, by means of 
microscopical observations, but no two observers have 
been agreed on the point. It is fortunate that it is not 
important to settle it : it is sufficient to know respecting 



INTRODUCTION. 23 

the structure of the muscles, that they are made up of 
fibres. 

Another subject which has occupied the attention of 
Physiologists, is, how for muscular contractility is in- 
dependent of nervous power. It would be foreign to 
the character of a work so purely elementary as this, to 
enter at all into the examination of that question. 



Of the Osseous System. 

The bones are the most solid part of the body. 
Their uses in the animal economy are various. They 
form as it were a frame for the other parts, giving firm- 
ness to the whole: in conjunction with the muscles, 
they are the agents of locomotion, and they enclose and 
protect some of the most important organs of the sys- 
tem. In man, and in all the higher orders of animals, 
they are in the interior of the body, whilst in lobsters, 
crabs, &c., they are on the outside, forming a case which 
protects the other more delicate parts from injury. 

Bone is composed in part of earthy matter, and in 
part of animal matter. By subjecting it to the action 
of some of the acids, the earthy part is removed, and 
nothing remains but the animal, yet the form of the 
bone is unchanged. By exposing it to the action of a 
moderate fire an opposite effect is produced, the ani- 
mal part is removed and the earth remains, with but 
slight alteration in its texture. 

The earthy part of bone has been ascertained to be 
the phosphate of lime, a substance of a very indestruc- 
tible nature. The bones of animals have been found. 



24 INTRODUCTION. 

which apparently have undergone but little change in 
the revolutions of ages. Geologists teach us, that the 
bones of animals which perished at periods, of which 
there is no record, are still in existence. 

The animal part of bone was supposed to consist, till 
recently, of gelatine; but Mr. Hatchett, of Great Britain, 
has proved that it is condensed albumen. Gelatine, no 
doubt, exists in the bones of young animals in large 
quantity, and is easily obtained from them by boiling. 

Albumen, in a pure liquid state, is found in abun- 
dance in the white of an egg, and from this circum- 
stance it derives its name. In a solid form it consti- 
tutes the principal part of hair, nails, and horn. It is 
insoluble in water, alcohol, or oils, but is dissolved 
by the alkalies. In a liquid state, it becomes hard by 
the action of heat, acids, alcohol, and some other sub- 
stances. 

Gelatine, in a liquid state, is known by the name of 
jelly; in a solid state by that of glue. It is soluble in 
hot water, the acids and alkalies, but is insoluble in al- 
cohol, ether, and oils. It has been supposed to form 
the principal part of the cellular texture, the skin, car- 
tilages, and hgaments ; but this opinion has recently 
been called in question. 

The bones are supplied with blood-vessels, and 
covered by a delicate membrane, called periosteum. 
They have but kw^ if any, nerves; certainly none of 
sensation, as they are destitute of sensibility in health. 
In amputating a limb, the patient makes no complaint 
while the surgeon is sawing through the bone, if it be in a 
sound state; when diseased, however, it is exquisitely 
sensible. 



INTRODUCTION. 25 

The bones, like all other parts of the body, are 
formed from the blood. The animal or albuminous 
part is first deposited in the exact form of the bone, 
and this serves as a mould to receive the earthy matter, 
which is afterwards added. 



Of the Fibrous System. 

The fibrous system is found in various parts of the 
body. It forms the tendons, which are the termination 
of the muscles, the hgaments which connect the bones, 
and thus form the joints, the dura-mater, the strong 
membrane, which covers the brain and the periosteum, 
or the covering of the bones. It enters also into the 
composition of many other organs of the body. 

Though arranged in different forms, it every where 
possesses the same properties. It is of a dull, white 
color, bordering on grey; of great strength and power 
of resistance, with a slight degree only of elasticity; and 
destitute, in great measure, of contractility and sensibility. 
Neither absorbent vessels nor nerves have been discov- 
ered in it. In some parts of the fibrous system, as the 
dura-mater, blood-vessels are very abundant, while 
others seem to be destitute of them. 

This concise account of six of the simple systems, 
will perhaps render more intelligible the descriptions 
which will hereafter be given of some of the organs. 
Some of the five remaining ones will be spoken of, 
when treating of the functions with whose organs they 
are most connected : thus the nervous system will be 
3 



26 INTRODUCTION. 

described, when we come to speak of the sensations 
and their organs, the brain and nerves; and the vascu- 
lar will come under notice, in the account that will be 
given of the circulation, and the agents by which it 
is carried on, the heart and blood-vessels. 

In treating of the various functions of the body, those 
will be first examined which are connected with the 
nourishment and growth of the individual, and after- 
wards those which connect him with external objects. 
Among the most important of the first class is digestion. 



(27) 



CHAPTER I. 



OF DIGESTION. 



Digestion is an important part of that process 
by which aliment taken into the body is made 
to nourish it. The apparatus by which it is ac- 
complished is of a very complicated kind. It is less 
so in those animals who hve on substances simi- 
lar to their bodies, as in the carnivorous animals, or 
those that feed on flesh, than in those that subsist on 
substances of a dissimilar character, as in the herbivor- 
ous animals, or those that live entirely on vegetables. 
Man can derive nourishment from almost every article 
of food, and on this account he has been called an om- 
nivorous animal. His digestive apparatus is therefore 
less complex than that of the herbivorous animals, but 
more so than that of the strictly carnivorous ones. 



Of the Digestive Apparatus. 

This may be divided into the mouth and its appen- 
dages, the stomach and the intestines. 

In the human adult, the mouth contains thirty-two 
teeth. There are four incisor or cutting teeth, six 



28 DIGESTION. 

canine, and six molar or grinders, in each jaw. There 
is another set of teeth, consisting of twenty, which begin 
to appear in the course of the first year after birth, all of 
w^hich are usually cut by the time the child is two 
years and a half old, and which ordinarily become loose at 
about seven years of age ; they then fall out, or are 
easily removed, in most instances, and give place to the 
second set. Some of the second set sometimes appear 
even earlier than this. 

The internal part of the teeth has a strong resem- 
blance to bone, but they have an external covering of a 
somewhat different character, called enamel. This 
substance is very hard, highly polished, insensible, and 
widiout any appearance of organization. If a portion 
of it be removed in any way, it is not replaced. The 
formation of teeth is not similar to that of the bones. 

The shape of the teeth is adapted to the kind of 
food, on which the animals to whom they belong sub- 
sist. This is so universally true, that it has been 
adopted as one of the most striking characteristics in 
some of the classifications that have been formed of the 
animal kingdom. 

The jaws perform an important part in rendering the 
food fit to be conveyed into the stomach. The upper 
jaw is firmly united to the bones of the face and head, 
and has no motion independent of them. The lower 
jaw is connected to those bones by ligaments and mus- 
cles, and has a joint, which allows it to perform motions 
of great importance. It moves not only directly up- 
wards and downwards, so as to bring its teeth in con- 
tact with those of the upper; but it also moves laterally, 



DIGESTION. 29 

producing an effect like grinding, which crushes almost 
every article of food which comes between the teeth of 
the two jaws. The muscles inserted into the lower 
jaw, are of great strengdi ; by their action alone, some 
persons are enabled to bite the hardest substances, and 
even to crack nuts with strong shells. 

The tongue also assists in the process of mastication. 
It removes the food from one part of the mouth to 
another, and forms it into the most convenient shape. 
It is a muscular organ and possesses great power. In 
addition to the nerves which give it the sense of 
taste, it is supplied with a pair of very large nerves, 
called the ninth pair, with apparently no other purpose 
than to increase its muscular energy. 

Three pairs of glands pour the fluid w^hich they se- 
crete into the mouth. This fluid is called saliva, and 
is an important agent in the first part of the process of 
digestion. The largest of these glands are called paro- 
tid^ from being situated about the ear. The tube 
which conveys their secretion into the mouth, pierces 
the muscle in the cheek, and has been called the duct 
or canal of Steno, from an anatomist who described it. 
It is this gland which is the seat of the disease known 
by the name of the Mumps. The sub-maxillary glands 
are situated under the lower jaw, and the sub-lingual 
under the tongue. An obstruction in the duct of one 
or both of the sub-maxillary glands, produces that 
singular affection which is called ranula or frog-tongue. 
It consists of a swelling, sometimes of the size of a pi- 
geon's egg, resembling a bladder, situated directly under 
the tongue. It is not unfrequently met with in chil- 
3* 



30 



DIGESTION. 



dren. It is computed that no less than eight ounces of 
saliva are poured into the mouth, from these different 
glands, at every full meal. 




a Parotid Gland, b Duct for conveying the saliva 
into the mouth. 

In immediate connexion with the mouth, and situated 
in the posterior part of it, is the pharynx, so called, 
which is the commencement of the passage to the sto- 
mach. It is of an irregular form, being considerably 
larger at its upper, than at its lower extremity. Its con- 
tinuation is called oesophagus, which is of a cylindrical 
shape, and of nearly an uniform size. It extends from 
the pharynx to the stomach. Both of these passages are 
lined by the mucous membrane, which is surrounded by 
muscular fibres, and supplied with nervous filaments and 
blood-vessels. It is by the means of the pharynx and oeso- 
phagus that the act of deglutition or swallowing is effected. 

The stomach is the largest organ of digestion. It is 
of an irregular shape, and closely resembles the bag of 



DIGESTION. 31 

a bag-pipe. It lies directly across the body, just under 
the diaphragm or great muscle which separates the 
chest from the abdomen. The oesophagus enters it in 
the upper part of the left extremity, whi h is much 
larger than the right. The opening into the stomach 
is called the cardiac orifice, and the opening from it 
into the intestines is called the pyloric orifice. The 
stomach of an adult in an ordinary state of distention, 
is capable of holding about three pints. It was for- 
merly described as consisting of many coats; each of 
the different layers of cellular membrane which exist 
between the coats, being considered as a distinct one. 
Exclusive of the cellular coat, it has three. The first 
is the mucous coat which lines it : this coat contains a 
great nnmber of small glands, by which not only the 
ordinary mucus is secreted, but which are supposed to 
furnish the gastric juice, a very important agent in di- 
gestion. Immediately under the mucous coat, numer- 
ous muscular fibres are found, some of which run in a 
longitudinal direction, and others in a transverse; and 
these form the muscular coat. Exterior to this is the 
serous coat, forming a smooth and highly polished 
covering. 

The stomach is abundantly supplied with blood-ves- 
sels and nerves, — an evidence of the important part it is 
designed to perform in the vital functions. Its nerves 
are derived from a variety of sources; some from what 
are called the ganglions; others from the spinal marrow, 
and what is still more remarkable, a pair is sent from 
the brain itself. 

At the pyloric orifice of the stomach, there is a 



32 



DIGESTION. 



membranous fold, which acts something like a valve, 
and which is supposed to be capable of preventing the 




The Stomach, a CEsophagus. b Cardiac portion, 
c Great or Jeft extremity, d Small extiemity, e 
Stomach, tied at the pylorus. / Great anterior 
curvature, g Omentum or caul. 

exit of its contents till they have been sufficiently acted 
upon, for the purposes of digestion. 

The intestines in the human subject are usually 
from six to eight times as long as the individual to 
whom they belong. They are divided into small and 
large intestines, the former constituting about four-fifths 
of the whole. The small intestines are divided into the 
duodenum, the jejunum, and the ileum. The duodenum 
is so called from its length being about equal to twelve 
fingers in breadth. It is somewhat larger than the other 
two. The ducts from the pancreas and fiver enter into it, 
usually at about one-third of the distance between the 



DIGESTION. 33 

Stomach and the jejunum. Sometimes they enter sep- 
arately and sometimes by a common opening. The 
duodenum is abundantly supplied with absorbent ves- 
sels, called lacteals, from the resemblance of the fluid 
they contain to milk. 

There is no marked difference between the jejunum 
and the ileum. On the mucous surface of both, there 
are small follicles, called the glands of Peyer, Brunner, 
and Lieberkuhn, and they both contain many absorbent 
vessels; they are not so numerous, however, as in the 
duodenum. 

Where the ileum enters the large intestines, there is 
a valve, so as to prevent the return of the intestinal 
contents into the stomach. All the intestines are sup- 
plied with three coats, similar to those of the stomach. 

Of the Digestive Process. 

Mastication and deglutition are intimately connected 
with digestion. As soon as the food is taken into the 
mouth, it is cut by the incisor teeth, or ground by the 
molar ones, according to the nature of it, into minute 
parts, and in this way becomes intimately mingled with 
the saliva. The pow^erful contraction of the muscles 
which move the lower jaw, in mastication, increases the 
flow of this fluid. A large quantity of saliva, is at all 
times poured into the mouth, but so long as we swallow 
with ease, we are not aware of the amount of the secre- 
tion. But this becomes evident the moment the throat is 
inflamed, or deglutition is difficult from any cause. The 
presence of savory food in the mouth, increases to a 



34 DIGESTION. 

great degree the action of the salivary glands, and if 
mastication be properly performed, every particle of 
food is completely surrounded with saliva. It is then 
carried by the action of the tongue and the muscles of 
the mouth into the pharynx. This part of deglutition 
is voluntary; but the passage of the food through the 
pharynx is wholly involuntary, and is performed very 
rapidly, so that no part of it may enter the wind- 
pipe, over which it passes. 

To prevent the introduction of any portion of the 
food into the air-passage, a very simple, and, at the same 
time, a very effectual contrivance has been adopted. 
There is attached to the root of the tongue, a small 
cartilaginous body, called epiglottis, as it is intend- 
ed to cover, under certain circumstances, the glot- 
tis, or opening into the larynx, the organ of voice. 
The ordinary position of this little organ, is perpendicu- 
lar, so as not to obstruct the passage of air to the lungs. 
But in the act of swallowing, the tongue is carried 
backwards, and the epiglottis is brought directly over 
the glottis, so as to completely close it. It remains in 
this state till the food has passed over it, and it is then 
restored to its ordinary position by the relaxation of 
tlie muscles, and its own elasticity. As the passage of 
the air to the lungs cannot with safety be long inter- 
rupted, it is necessary that this part of swallowing 
should be, as we see it is, rapidly performed. 

As soon as it enters the oesophagus, it is carried 
slowly downw^ards towards the stomach. The muscular 
contraction by which it is accomplished, is powerful, 
sufficient to overcome a very considerable degree of 



DIGESTION. 35 

resistance. It then enters the stomach, and there un- 
dergoes a change, by which it is brought into a homo- 
geneous mass, neither fluid nor sohd, which is called 
chyme. There has been great diversity of opinion 
among physiologists as to the mode in which this was 
effected. 

Hippocrates, who made the first attempt to explain 
the digestive process, believed that the food was con- 
verted into chyme, by what he called coction. This 
opinion was adopted by Galen, and prevailed for some 
centuries. It is probable that he attached to the word 
coction no very precise meaning, but used it merely as 
a substitute for that of digestion. If, however, he in- 
tended to imply by the term, that anything like boiling 
went on in the stomach, he obviously labored under a 
very great mistake. A decisive objection to this no- 
tion, is that the temperature of the body is not sufficiently 
high; and even if it were, the effect produced on the 
food by the stomach, is not similar to that arising from 
boiling. It is probable, however, that the ancients em- 
ployed the term coction in explaining digestion, to veil 
their ignorance of the subject, without giving to it the 
precise meaning that some of their followers have since 
attempted to do. 

The next theory of digestion that attracted much 
notice, was that which taught that the food underwent 
a putrefactive process in the stomach. The facts in 
opposition to this are unanswerable. There are many 
birds and other animals of prey, that feed on carrion in 
the most putrid state, and it has been repeatedly ascer- 
tained, that when flesh has been taken into the stomach 



3d digestion. 

in this state, it becomes perfectly sweet after remaining 
there some time. 

A bird in attempting to swallow a pike, has died by 
choking. When found in this situation, that part of the 
fish which remained in the throat was putrid, while that 
which had entered the stomach was wholly untainted. 
No one in modern times has given the slightest coun- 
tenance to this theory, with the exception of Cheselden 
the celebrated English anatomist. 

The next theory that was brought forward was a 
mechanical one^ and all the phenomena of digestion 
were accounted for by trituration. This notion was 
first suggested, no doubt, to the supporters of the doc- 
trine, by the known action of the gizzard in some spe- 
cies of birds, and a supposed analogy between the of- 
fices of this organ and those of the human stomach. 
But the fact is, that the gizzard does not correspond to the 
stomach, but merely takes the place of the teeth, those 
animals who are furnished with a gizzard being destitute 
of them. This circumstance, however, was overlook- 
ed by the disciples of this theory, and nice mathemati- 
cal calculations were made to show the prodigious 
power with which the stomach could act upon its con- 
tents. It is really amusing to see how widely different 
were the results at which these calculators arrived ; 
one of them estimated the muscular power of the stom- 
ach to be equal to 12,951 pounds ; anodier thought it 
to amount to about 20 pounds ; and a third reduced it 
down to five ounces. 

They chose also to overlook a well-known fact, that 
while many hard and tough substances are easily di- 



DIGESTION. 37 

gested, the pulp of a grape will very often at the same 
time pass through the stomach without undergoing any 
change. 

Some experiments of Abbe Spallanzani and others, 
which will be particularly spoken of hereafter, render 
it certain that food can be digested in the stomach with- 
out the slightest mechanical pressure of that organ. 

When the mechanical theory of digestion was aban- 
doned, another, which may be called a chemical one, 
look its place. This attributed the whole process to 
fermentation. If this were true, food should undergo 
the same change out of the stomach, when subjected 
to the same degree of heat and moisture, that it does 
in. But this is not the case ; the chemical laws seem 
to be controled and modified by the laws of the living 
body. 

The doctrine that was supported by the celebrated 
Haller, maintained, that digestion w^as effected by ma- 
ceration, the food being confined in the stomach, and 
subjected to an uniform temperature, and constantly im- 
mersed in fluid. There are two objections to this ; the 
first is, that maceration is a much slower process than 
digestion ; and the second is, that in maceration the na- 
ture of the substances macerated remains unchanged, 
which is not the case in digestion. 

The opinion that is now most generally received re- 
specting the mode in which the stomach acts on the 
food that is taken into it, is, that a peculiar liquor se- 
creted by the stomach, and called gastric juice, has a 
solvent power, which enables it to reduce the food to an 
uniform .mass. This is sometimes called the theory of 
4 



38 DIGESTION. 

chemical solution. This solvent power seems to have 
been satisfactorily proved by the experiments of the Ab- 
be Spallanzani, which were before referred to. He be- 
lieved that the gastric juice varied in different animals 
according to the nature of the food on which they fed ; 
that that contained in the stomachs of animals strictly her- 
bivorous, differed from that of the carnivorous animals. 
He obtained quantities of it, at different times, fi'om va- 
rious animals, and subjected it to very minute examina- 
tion: he denied that it was either acid or alkaline ; but no- 
thing satisfactory is yet known of its nature: it has been 
repeatedly examined since his time by acute observers, 
and no two have arrived at the same conclusions. 

The solvent power of the gastric juice, he proved by 
many experiments. He caused animals to swallow 
tubes with holes in them, containing food which had 
been previously chewed, and he found that it was con- 
verted into chyme. 

He tried similar experiments on himself. He at 
first swallowed wooden tubes, into which he put various 
articles of food ; but as these produced pain in the 
stomach, he substituted linen bags, and found that their 
contents were invariably digested. This he attributed 
to the solvent power of the gastric fluid, which pene- 
trated the linen, and dissolved the food. As some me- 
chanical pressure might have been made on the bags in 
this case, Dr. Stevens of St. Croix, then in Edinburgh, 
devised an experiment, which obviated this objection. 
He caused some hollow metallic balls to be made, per- 
forated with holes, and into these balls he Introduced 
food that had been masticated. These were swallow- 



DIGESTION. 39 

ed, and, when subsequently examined, were found en- 
tirely empty. 

It is therefore certain that the gastric juice, while in 
the stomach, is capable of dissolving food taken into it. 
But Spallanzani ascertained, to his own satisfaction at 
least, something further. He mixed some masticated 
food with the liquor of the stomach in a glass tube, and 
placed it in his arm-pit, that it might have about the 
same temperature as if it had been in the stomach. 
At the expiration of a number of hours, varying in 
his different experiments from fifteen to forty-eight, he 
found, he says, that the food was converted into chyme. 

This experiment, however, has been often repeated 
since, but the same result has never been obtained. 
The food not only has not been converted into chyme, 
but the gastric juice employed in the experiment has 
frequently become putrid. This is what might have 
been expected; for it is difficult to believe that the pro- 
cess of digestion is dependent on chemic€il laws alone: 
on the contrary, it can hardly be doubted that these are 
controled by the laws of vitality, which preside over 
the functions of the animal economy. 

The gastric juice, though evidently possessing a 
strong solvent power, seems to be incapable of acting on 
anything endowed with life. Worms, while living, 
will often remain in the stomach uninjured, at a time 
when the hardest substances are undergoing solution ; 
but the moment these animals are dead, they are dis- 
solved by the gastric liquor. 

Mr. Hunter has stated some remarkable facts in re- 
lation to the solvent power of this fluid. In some per- 



40 DIGESTION. 

sons, who have died suddenly without previous disease, 
and after long fasting, the stomach has been found perfo- 
rated with numerous holes. The only satisfactory expla- 
nation seems to be, ' that the stomach itself is partially 
digested by the gastric juice, which had been previous- 
ly secreted.' 

The gastric liquor has also the property of rendering 
solid, or coagulating, as it is termed, liquid albuminous 
matter, when it is mixed with it. On this property de- 
pends the process by which cheese is made. Rennet, 
which is employed for this purpose, is an infusion of 
the fourth, or digestive stomach of the calf, and Dr. 
Fordyce ascertained that six or seven grains of the ' in- 
ner coat of this stomach, after being infused in water, 
afforded a liquid, which coagulated more than one hun- 
dred ounces of milk.' It has been ascertained, that 
milk, and all other albuminous fluids, are coagulated in 
the stomach by the gastric liquor; and the appearance 
of curd, in what is sometimes thrown from that organ 
after milk has been taken into it, is no indication of 
disease, as the coagulation of milk, or its formation into 
curd, is the first step in the process of digestion. 

No change, except a mechanical one, takes place in 
the food in the first hour after it has been conveyed into 
the stomach. During that period, it becomes intimately 
mixed with the gastric juice, which seems to be an es- 
sential preliminary step in the digestive process. This 
is easily and thoroughly effected if the food has been 
properly masticated; but it cannot be readily done, un- 
less we eat slowly, and chew the food sufficiently long 
to divide it into very minute parts. 



DIGESTION. 41 

The introduction of food into the stomach, produces 
an increased secretion of the gastric liquor, which is 
poured out in such abundance in health, as to surround 
every particle of it. When this is accomplished, 
an alternate contraction and expansion of the stomach 
takes place, and continues till the whole alimentary mass 
is converted into chyme. This motion is produced by 
the muscular coat of the stomach, which is formed, as 
has been before stated, by muscular fibres running in a 
longitudinal and transverse direction. 

It has been ascertained, that in a healthy stomach, 
the food, if easy of digestion, is converted into chyme, 
in four or five hours, and that before this change has 
taken place, it is prevented from passing into the intes- 
tine, by a sort of valve situated at the pyloric orifice of 
the stomach, called pylorus, or door-keeper. It has 
been supposed by some, that this valve has the property 
of determining when the aliment was sufficiently changed 
to allow it to pass, that it gives free exit to chyme, and 
contracts when undigested substances attempt to enter 
the duodenum. 

The food is not all converted into chyme at the same 
time; but as fast as it is changed, it passes into the in- 
testine, only two or three ounces collecting in the py- 
loric extremity at once. 

The change which the alimentary mass undergoes in 

the first intestine or duodenum, as it is called, is as 

great and important as the one which is effected in it in 

the stomach. In that organ, it is converted into chyme, 

and the process is called chymification: in the intestine, 

it undergoes what is called chylification; in which, it is 
4* 



42 DIGESTION. 

brought into such a state, that a peculiar fluid, called 
chyle, can be extracted from it by tlie absorbent vessels, 
whose mouths open in great abundance into this intes- 
tine. This chyle is a thin milky fluid, and these absorb- 
ents are thence called lacteals. 

The chyme passes slowly through the duodenum, 
and in its passage it becomes intimately mixed with the 
liquor secreted by the pancreas or sweetbread, and the 
bile which is formed by the liver. These fluids some- 
times pass through separate tubes, and at others enter 
by a common canal. The inner coat of the first intes- 
tine is covered with folds of its lining membrane, which 
answer the purpose in some measure of valves, retard- 
ing to some extent the passage of the chyme, and pre- 
venting, under ordinary circumstances, its regurgitation. 
In this way, the absorbents have an opportunity of sepa- 
rating from it the chyle, the fluid which is afterwards to 
be converted into blood, for the nourishment of the 
body. 

The chyle has frequently been examined, with a 
view of ascertaining its nature and properties. It has 
no inconsiderable resemblance to cream in appearance, 
and when removed from the body and sufl^ered to stand, 
it undergoes a species of spontaneous coagulation. It 
separates into three parts, a transparent and colorless 
fluid, a firm and white coagulum, and a thin pellicle of 
fatty matter, which floats on the surface; a process not 
unlike that, which will be hereafter spoken of, as taking 
place in the blood when removed from the body. 

It has also been found, upon analysis, that its constit- 
uent principles resenible those of the blood, in some 
of their properties. The same salts are found both in 



DIGESTION. 43 

the chyle and the blood; but the chyle contains a very 
considerable quantity of fatty matter, which rarely, if 
ever, exists in the blood. 

Another point of resemblance between these two 
fluids, is the existence of globules in the chyle, which 
has been ascertained by microscopical observations, and 
which resemble the red globules of the blood. 

The chyle differs somewhat, according to the food 
on which the animal from whom it is obtained, has sub- 
sisted. If the food has contained a considerable por- 
tion of fat, the chyle will be of a white milky appear- 
ance; but if it contained a very small quantity of it, it 
will be semi-transparent, and a thinner pellicle will form 
on the surface when it is permitted to coagulate. 

The chyle is not changed in color, by the color of 
the food on which the animal is fed: a different opinion 
has been maintained, but the point has been fully deter- 
mined by numerous experiments. 

It is not satisfactorily settled how the process of chy- 
lification is effected. It seems to be probable, how- 
ever, that the liquor from the pancreas and the bile are 
important agents in the process. It has been ascer- 
tained, that the chyme is not changed till it reaches the 
orifices of the pancreatic and biliary tubes. Mr. Bro- 
dies tied the duct coming from the liver, in a living 
animal, which he says stopped the process of chylifica- 
tion: the experiment was repeated by Mr. Magendie, 
without producing the same effect. It is, however, 
certain that no chyliferous vessels can be discovered in 
the small intestine above the place where the ducts 
from the pancreas and liver enter. The chyle that is 



44 DIGESTION. 

taken up by the lacteals, is conveyed through vessels 
appropriated to the purpose, into the blood. The 
course which it takes, and the agents by which it is ef- 
fected, will be described when w^e speak of absorption. 
Before doing this, how^ever, it will be proper to give 
some account of the circulation of the blood, the circu- 
lating apparatus, and the nature of the blood itself. 



(45) 



CHAPTER II. 

OF THE CIRCULATION OF THE BLOOD. 

The agents by which the circulation of the blood is 
carried on, are the heart, the arteries, and the veins. 
The heart is a hollow organ, of an irregular conical 
shape, and of a muscular and fibrous structure. It 
is situated in the anterior part of the cavity of the 
chest, inclining towards the left side. It rests on 
the diaphragm, the muscle which separates the chest 
from the abdomen, and it is supported at its base, which 
is uppermost, by the large blood-vessels with which it 
is connected. It is enclosed in a strong bag, called 
pericardium, which is attached below to the diaphragm, 
and above to the great arteries and veins, which go out 
of and come into it. The heart is covered on its exte- 
rior by a serous membrane, and the pericardium is lined 
with the same. Within the pericardium, there is usually 
found after death a small quantity of serous fluid, varying 
from an eighth to half an ounce, which is supposed to be 
in the form of vapor during life. 

The heart in man is a double organ, and there is no 
direct communication between the two sides after 
birth. It is by one of these sides, that the circulation 
is carried on in the lungs, and by the other throughout 



46 



CIRCULATION OF THE BLOOD, 



tlie rest of the body. They are usually called the 
right and left sides, though it would be more proper to 
call them anterior and posterior, or designate them by 




Tho cavity of the cliest laid open, to show tlie heart and lungs, o 
The heart, bh The pericardium, cut open, c The aorta, the 
great artery of the lefr side, that distributes ihe blood to nil parts 
of the body, d The great Aein, called the descending vena cava, 
which, with the ascending, brings the blood to the rigiit auricle. 
6 The pleura or membrane that covers the lungs. 

the names of the parts with whose circulation they are 
connected. The error arose with the ancients, whose 
prejudices did not allow of human dissection, and who 



CIRCULATION OF THE BLOOD. 



47 



obtained their ideas of the circulating organs by the 
dissection of monkies, in whom the heart is somewhat 
differently placed than in man. 

The anterior or right side of the heart, as it is called, 
sends the blood to the lungs; the posterior, or left side, 




O The left ventricle, h The right ventricle. C «/ The 
aorta, the great artery that goes off (rom the left ven- 
tricle, g ki The arteries that are sent from the arch 
of the aorta. A; The pulmonary artery, that goes from 
the right ventricle to the lungs. I I Hranclios of the pul- 
monary artery, going to the two sides of the lungs, m m 
The pulmonary veins, which bring the blood back from 
the lungs to the left side of the heart, n The right auri- 
cle. The ascending vena cava, q The descending : 
these two meet, and by their union, form the rigiit auricle. 
p The veins from the liver, spleen and bowels, s The 
left coronary artery ; one of the arteries which nourish the 
heart. 

sends it to the rest of the body. Each side of the 
heart is divided into two parts ; the uppermost, which 
is a membranous bag, is called, from its resemblance to 



4S CIRCULATION OF THE BLOOD. 

an ear, auricle. Though membranous, it is not en- 
tirely destitute of muscular fibres, and is consequently- 
possessed of some contractile powers. The auricle 
communicates directly with the lower part, which is 
much thicker, and composed chiefly of strong muscu- 
lar fibres: this is called the ventricle. 

In the human heart, therefore, there are four cavities, 
two auricles, and two ventricles. The auricles are situ- 
ated at the upper part, and communicate freely with the 
ventricles. The auricle and ventricle of one side are 
separated from those of the other by a septum, through 
which there is no communication in a healthy individual 
after birth ; before that the blood passes from one auri- 
cle to the other. The right or anterior auricle is some- 
what larger than the left, being in the proportion of 
about seven to five. The cavities of the two ventri- 
cles are nearly of a size, but the parietes or walls of the 
left are much thicker and more powerful than those of 
the ridit. The auricles receive the blood from the 
veins, and transmit it immediately into the ventricles : 
when the blood, thus transmitted, enters the right ven- 
tricle from the auricle, the ventricle contracts and 
throws its contents through a system of arteries, 
destined for the purpose, to the lungs. The blood is 
prevented from returning to the auricle by valves which 
open towards the ventricle. A similar arrangement ex- 
ists on the left or posterior side of the heart ; the blood 
passes from the left auricle to the left ventricle, and is 
thence sent to all parts of the body. 

The vessels which convey the blood from the heart, 
are called arteries. This name was given to them by 



CIRCULATION OF THE BLOOD. 49 

the ancients from a belief that they contained air. They 
are generally described as having two coats, the outer 
one being almost entirely composed of cellular mem- 
brane, is called the cellular coat, and the inner one, 
which is much firmer and very smooth, partakes of the 
nature of the tendons, and is evidently of a fibrous 
structure. Between these two coats, there are some 
transverse fibres, which seem to be of a muscular char- 
acter, and which have sometimes been called the mus- 
cular coat. These fibres give the arteries a contractile 
power. 

At the mouths of the two great arteries that go off 
from the heart, there are three valves in each, which 
prevent the blood that enters the arteries, from return- 
ing to the heart. There are no other valves found in 
any part of the arterial system. 

The arteries are strong elastic tubes, of somewhat of 
a conical shape, and of a white color. Their firmness 
and elasticity have been made very apparent in some 
cases of severe injury. Accidents not unfrequently 
occur, in which the bones of a limb will be broken, the 
muscles or flesh torn through, and yet the artery, 
though stretched to a great degree, remains whole and 
apparently uninjured. 

The veins are the vessels by which the blood is 
conveyed to the heart. They differ from the arteries 
in being thinner and more delicate, of a less white color, 
and possessing but few, if any, of the transverse mus- 
cular fibres. There is another difference too, of some 
importance, which is, that many of the veins, particu- 
larly in the extremities, are furnished with valves. 
5 



50 CIRCULATION OF THE BLOOD. 

It must be apparent, even to a casual observer of 
the vital phenomena, that the blood is in perpetual motion 
during life. This was familiarly known to the ancients, 
but they were wholly ignorant of the course w^hich the 
blood took, and the means by which its motion was 
kept up. They believed that the arteries contained 
air, and that the blood was to be found only in the veins. 
These opinions, with but slight variation, were preva- 
lent till the seventeenth century. The discovery of 
the circulation of the blood, was made by Dr. William 
Harvey, an English physician, in the year 1620, though 
he did not make it public till eight years after. Serve- 
tus, better known as a theologian, and one or two others 
before Harvey's time, made some steps towards this 
important discovery, but not enough to diminish in the 
least degree, the glory to which the discoverer of a fact 
of such incalculable importance is justly entided. When 
he first made his discovery public, he adduced in sup- 
port of it such facts and arguments, that it seems almost 
incredible at the present day that it was not at once ac- 
knowledged. But so far from this being the case, that it 
was at the time regarded with distrust by individuals of all 
descriptions, his professional business was sensibly di- 
minished by it, and it has been asserted, that there was 
not a physician, who was forty years of age, at the 
period of its promulgation, who ever became a convert to 
the doctrine. Notwithstanding all these discouraging 
circumstances, Harvey lived to see the truth of his doc- 
trine universally admitted, and to reap some of the 
fruits of his splendid discovery. 

Though no one at the present day pretends to deny 
that the blood passes from the heart through the arteries 



CIRCULATION OF THE BLOOD. 51 

into the veins, and is returned by them to the heart 
again; it may still be well to notice some of the facts 
and arguments that have been adduced in support of it. 
It is but justice, however, to Harvey to observe, that 
his first publication on the circulation of the blood con- 
tained nearly all the proofs of the fact that have ever 
been adduced. 

If the chest of a cold-blooded animal, (for animals of 
this class are the most tenacious of life, and can conse- 
quently bear such an experiment better than warm- 
blooded animals,) be opened, the heart will be seen al- 
ternately to dilate and contract. It then remains for 
an instant apparently at rest, the dilatation again com- 
mences, and when it has arrived at a certain point, 
contraction takes place. When the heart contracts to 
expel the blood, it rises up, as it were, on its base, and 
its apex strikes against the ribs, which produces what 
is called the beating of the heart. This has been sup- 
posed to be owing to the dilatation of the heart; but it 
is not so, — it is produced by its contraction; and this 
fact was known to Harvey. 

The valvular structure of the circulating system is 
a strong fact in favor of the received opinion as to the 
course of the blood. The valves situated between the 
auricles and the ventricles, allow the blood to pass 
freely from the former into the latter, but they ef- 
fectually prevent the blood from going from the ventri- 
cle to the auricle. The valves placed at the commence- 
ment of the arteries that go off from the heart, permit 
the blood to enter the arteries from the ventricles, but 
completely prevent it from returning to the heart. The 



62 CIRCULATION OF THE BLOOD. 

same may be said of the valvular structure of the veins; 
it allows the blood to go towards the heart, but not in 
the other direction. These are facts that are familiar 
to every anatomist. 

The mode in w^iich the common operation of bleed- 
ing from a vein, either in the arm or leg, is performed, 
is a convincing proof of the circulation of the blood. 
A ligature is placed around the arm or leg above the 
point at which the vein is to be opened. The blood 
is returning through it towards the heart, its passage is 
of course interrupted; the vein consequently swells, 
because the artery, which is deeper seated, is not com- 
pressed, and continues to carry the blood to it, and if 
the vein is then opened below the ligature, the blood 
flows freely; but no blood is obtained if an opening be 
made above the ligature. It sometimes happens to an 
inexperienced operator, to apply the ligature so tight, 
as to compress the artery; the consequence is, that 
after the vein is emptied, no more blood flows ; but if 
the ligature be removed, and reapplied less tightly, the 
vein will bleed again. 

The manner in which the bleeding from the vessels 
that are divided in surgical operations is stopped, is 
another proof of the circulation of the blood. In the 
amputation of an extremity, for example, that is, the 
removal of an arm or a leg, the surgeon ties only the 
arteries. These carry the blood from the heart to all 
parts of the body, and the patient would soon bleed to 
death, unless some means were adopted to prevent it. 
The veins, however, which carry the blood back to the 
heart, though they are usually as large as the arteries, 
do not bleed, and of course are not tied. 



CIRCULATION OF THE BLOOD. 63 

It has been said, that the circulation through the ar- 
teries and the veins, may be seen by a microscope in 
some dehcate parts, as in the web of a frog's foot. 
Malpighi first made this observation, and it has been so 
frequently repeated, that no doubt now remains on the 
subject. It may therefore be considered an additional 
proof of the circulation of the blood. Microscopical 
observations, however, when adduced to maintain any 
theory in physiology, and unsupported by other evidence 
should be received with caution, as the observers not 
unfrequently see precisely what they wish. 

With a knowledge of such facts as these, it is impossi- 
ble to resist the conclusion, that the blood passes from 
the heart through the arteries into the veins, and is re- 
turned by them again to the heart. The chyle, as has 
been before stated, when speaking of digestion, which 
is formed in the duodenum, is taken up by a set of ab- 
sorbent vessels, whose mouths open in that intestine, 
and which are called lacteals, and is thence conveyed 
into the blood. These absorbents unite in a common 
trunk, called the thoracic duct, which pours its con- 
tents into a vein, situated under the collar bone. This 
vein soon empties its blood into a large vein, which 
receives all the blood coming from the parts above the 
heart; another large vein receives all that which comes 
from the parts situated below that organ, and these two 
veins, which bring the blood from every part of the 
body except the lungs, pour their contents into the 
right, or anterior auricle. From the auricle the blood 
is immediately conveyed into the ventricle, probably by 
means of a slight contraction of those muscular fibres, 



54 CIRCULATION OF THE BLOOD. 

with which it is known the auricle is furnished. As 
soon as it enters the ventricle, a powerful contraction 
takes place, and the blood is driven with great force 
through the pulmonary artery into the lungs. When 
this contraction is made a portion of the blood 
would be forced back into the auricle, were it not for 
three valves called tricuspid valves, which are situated 
between the auricle and ventricle. The blood might 
also flow back from the artery as the contraction 
ceases, if there were not some mechanical means to 
prevent it, and these consist in three valves situated at 
the entrance of the artery, called semilunar valves. 

Between the termination of the arteries and the com- 
mencement of the veins, there exists not only in the 
lungs, but throughout the body, an intermediate set of 
vessels, which, from their minute size, are called capil- 
laries. It is in them that the vital functions of the cir- 
culating system are carried on, and it is in the capillaries 
of the lungs that that peculiar change takes place in the 
blood, which is effected by respiration, and which will 
be particularly described in treating of that function. 
The circulation of the capillary vessels is independent 
of the heart, but is carried on by the muscular power of 
the vessels themselves. The blood having passed 
through the capillary system of the lungs, and having 
undergone a remarkable change in its properties and 
color, being converted from dark purple to bright 
scarlet, is carried by the veins to the left or posterior 
auricle of the heart. It then goes, as on the other side, 
into the ventricle. This ventricle, which is far more pow- 
erful than the other immediately contracts, and throws the 



CIRCULATION OF THE BLOOD, 55 

blood into a great artery, which distributes it throughout 
the body. A similar arrangement of valves is found in this 
side of the heart, as has been described as existing in the 
other. There are three valves between the posterior or 
left auricle and ventricle, called mitral valves; and at the 
commencement of the aorta, or great artery, which 
goes off from this side of the heart, there are three 
more which are called semilunar valves. The first set 
of these valves prevents the blood from returning to the 
auricle when the ventricle contracts, and the second 
prevents that which is thrown into the artery from re- 
entering the ventricle, when its contraction has ceased. 

The blood, having passed through the arterial and 
capillary systems, enters the veins, and is carried by 
them to the anterior or right auricle of the heart, to go 
through again the course which has just been described. 

It was before observed, that in man, the heart is a 
double organ, and it now appears that there ai^e two 
distinct circulations, in some degree independent of 
each other. One of these, is that of the lungs, the 
other is that of the general system. The objects ef- 
fected by each are different, and will be spoken of here- 
after. 

There has been much diversity of opinion among 
physiologists upon the question, whether the blood is 
carried through the arteries by the action of the heart 
alone, the arteries being merely inert canals, or whether 
the arteries themselves act upon their contents, and thus 
aid in the circulation of the blood. The point may 
be still considered unsetded. A great majority, how- 
ever, incline to the opinion that the arteries are not 



66 CIRCULATION OF THE BLOOD. 

wholly inactive. They are known, especially the 
smaller branches, to possess muscular fibres, and of 
course they must have contractile power ; it has been 
ascertained in some cases, where an artery has been 
divided in a living animal, and has not been tied, that 
the divided artery has contracted to a considerable ex- 
tent. 

If the finger be placed with a moderate degree of 
force over some of the arteries, a pulsation will be felt, 
which corresponds with the contractions of the poste- 
rior or left ventricle of the heart; this is called the 
pulse. It was supposed, till recently, to be owing to 
a dilatation of the artery, produced by the entrance of 
the blood into it. It was doubted by Bichat, whether 
any dilatation of the artery took place, and Dr. Parry 
satisfied himself by repeated observations that it does 
not, and ascribes the pulse to the ' impulse of dis- 
tention ' given by the contraction of the ventricle. A 
more recent observer, however. Dr. Hastings, main- 
tains that in repeating Dr. Parry's experiments, he no- 
ticed a dilatation of the arteries. It is perhaps right, 
therefore, to attribute the pulse to the combined action 
of these two causes, viz. the impulse of distention im- 
parted by the ventricle, and a slight degree of dilatation 
of the artery itself. 

The blood circulates through the capillary system by 
the action of the capillaries alone; they do not feel the 
influence of the heart. In the pulmonary circulation, 
it is in the capillaries that the change in the blood 
which is effected by the air is WTOught; and in the cir- 
culation of the general system, it is in them that the 



CIRCULATION OF THE BLOOD. 67 

process of nutrition goes on. The blood having parted 
with such portions of it as are necessary to support the 
system, the remainder is conveyed by the capillary 
vessels to the veins, with a change in its color and 
properties. 

There is some diifference of opinion as to the means 
by which the blood is carried through the veins. Har- 
vey supposed that it was done by the action of the 
heart, which contracted wnth sufficient power to send 
this fluid through its w^hole course. This hypothesis 
has no advocates at the present day. It is believed by 
physiologists, that the circulation in the capillary ves 
sels is independent of the action of the heart, and if this 
be true, this organ can of course have no power over 
the circulation in the veins. 

Bichat thought that the venous circulation was car- 
ried on by means of the capillaries; that these vessels 
not only circulated the blood which they contained and 
conveyed it into the veins, but that they also carried it 
through these vessels to the right side of the heart. He 
considered them, in fact, as an agent of impulse, placed 
on the circumference, w^hich corresponded to the agent 
of impulse, the heart, which is placed at the centre. 

Other physiologists have supposed that the blood was 
circulated through the veins on the common principle 
of hydraulics, that a fluid contained in tubes will rise to 
the height of its source. This explanation is too me- 
chanical to be admitted to its full extent ; though there 
can be but little doubt that the gravity of the blood 
must exert some influence upon its circulation through 
the veins, and it is probable, that those vessels them- 
selves take but little, if any, active part in it. 



68 CIRCULATION OF THE BLOOD. 

Some of those who have admitted the influence of 
the two last causes, have attributed something also to 
muscular contraction, which would act with greater 
power in some parts of the body than in others. When 
the muscles contract, they press upon the veins, and 
thus aid the blood in its passage towards the heart. It 
cannot go in the other direction, particularly in the ex- 
tremities, on account of the numerous valves that exist 
in the veins. The contraction of the muscles, so far 
as it has any influence on the arteries, tends to retard 
the circulation; it diminishes the diameter of the artery, 
and consequently offers an additional resistance to the 
action of the heart. But little effect of this kind, how- 
ever, is produced; partly because the coats of the ar- 
teries are much firmer than those of the veins, and are 
not, therefore, so easily acted upon; and partly because 
the arteries are so situated that they are less subject to 
the action of the muscles than the veins. 

When the situation of the veins is considered being 
placed some above, others beneath, and others again 
passing through the muscles, it is easy to believe that 
their coats, destitute in great measure of muscular fibres, 
would yield to the muscles in a state of contraction; 
and when they are thus compressed, it is obvious from 
tlieir valvular structure that the blood must be sent to- 
wards the heart. 

To the agents already enumerated as the supposed 
causes of the venous circulation, two others have been 
added, which are sometimes called the suction powers 
of the lungs and heart. It is believed, by those who at^ 
tribute any influence to these causes, that in inspiration 



CIRCULATION OF THE BLOOD. 59 

or the act of drawing the air into the lungs, the blood 
is also drawn into the right side of the heart, by the ap- 
proach as it were of a vacuum, produced by an en- 
largement of the cavity of die chest. 

The suction power of the heart has also received the 
name of derivation. When the ventricles have con- 
tracted and sent out the blood which they contained, 
tliey relax, and consequently become enlarged, and a 
vacuum would take place, if the blood did not flow in 
from the auricles. 

It may then be said, that the blood is carried through 
the arteries chiefly by the action of the heart, aided in 
a shght degree, perhaps, by the elasticity and contrac- 
tile power, which reside in the arterial coats; that the 
circulation through the capillary system is effected en- 
tirely by the capillaries themselves, and that the blood 
is thrown by them into the veins, and that it is carried 
through them, probably by the combined action of a 
part or the whole of the causes just pointed out. 

It has been estimated that two ounces of blood are 
thrown out of the heart at each contraction of the ven- 
tricle, and that this fluid constitutes about one-fifth part 
of the weight of the body. It follows, therefore, that 
the quantity of the blood is different in different individu- 
als according to the size of the body, varying in healthy 
adults from twenty-five to thirty-five pounds, and in some 
cases it is even more than this. Now suppose that 
there are seventy pulsations or contractions of the heart 
in a minute, the whole blood of the body, even allow- 
ing it to amount to thirty-five pounds, must pass through 
that organ in less than three minutes. And so impor- 



60 CIRCULATION OF THE BLOOD. 

tant is the action of the heart, that if it be suspended 
for a moment, death ensues. That this organ, con- 
structed apparently of such frail materials, and exposed 
to such great irregularity in its action, should, during a 
long life, perform its functions so perfectly, is calculated 
to give us the most exalted ideas of the power and 
wisdom of the Creator of our bodies. 



Of the Blood. 

The blood is an adhesive fluid from which all the 
other parts of the body are formed. It is contained in 
the heart, the arteries, the veins, and the capillary sys- 
tem. When drawn from the body, it soon separates 
into two parts, the one liquid, and the other solid, 
which floats in it; the liquid is called serum, and the 
solid, crassamentum. This change is known by the 
name of the coagulation of the blood, and takes place 
on an average in about seven minutes after it is drawn 
from the vessels. The time, however, varies, partly 
from the manner in which the blood is drawn, and 
more from the temperature of the atmosphere in which 
the operation is performed; it coagulates quicker in a 
high temperature than in a low one. 

The temperature of the blood may be considered to 
be about one hundred degrees of Fahrenheit's thermom- 
eter. The blood of the veins is not perhaps quite so 
high as this, while that of the arteries is somewhat 
higher. 

It is not dasy to determine the exact proportion of 
the serum and the crassamentum, because some of the 



CIRCULATION OF THE BLOOD. 61 

former adheres so closely to the latter, that it cannot be 
separated from it. It is generally supposed, that the 
weight of the serum is three times greater than that of 
the crassamentum. 

When the crassamentum is taken out of the serum, 
it is found to be a soft solid, of sufficient consistence to 
be cut with a knife. If washed for some time in water, 
it loses its red color, showing that the red particles 
were only united to it by mechanical mixture. The 
solid thus obtained is of a dingy white color, elastic, 
of considerable firmness and tenacity, and of a fibrous 
structure. It is known by the name of fibrin. It is the 
same substance which is sometimes called by the different 
names of coagulable lymph, fibre of the blood, gluten, and 
adhesive matter. It constitutes the basis of the muscles 
or flesh. It is the substance which is poured out on all 
the serous membranes, when in a state of inflammation; 
and it is the means by which the union is effected in 
wounds. When any of the soft parts of the body are 
divided, fibrin is immediately effused from the cut 
ends of the blood-vessels, and thus glues, as it w^ere, 
the wound together. This fibrin afterwards becomes 
organized, blood-vessels shoot through it, and the part 
again resumes its healthy state. 

The other constituent of the crassamentum is known 
by the name of the red globules. These substances 
have afforded fruitful topics for conjee* ^re and physi- 
ologists have indulged in the wildest speculations con- 
cerning them. They early attracted attention, and 
when microscopical observations first became in 
fashion in science, they were subjected to very minute 
6 



62 CIRCULATION OF THE BLOOD. 

and faithful examination. The result was the formation 
of several extravagant hypotheses, to which no impor- 
tance is attached at the present day. 

It is impossible to obtain the red globules in a de- 
tached state, so as to subject them to any thorough and 
minute investigation; and this circumstance alone, will 
probably be sufficient to prevent us from knowing much 
more of them than is at present known. Berzelius, the 
celebrated Swedish chemist, is one of the latest, who 
has paid much attention to the subject. The conclu- 
sions to which he has arrived, and which no one seems 
disposed to controvert, are, that the red globules do not 
materially differ from the other parts of the blood ex- 
cept in the color, and the fact that a certain quantity of 
the red oxide of iron is found in their ashes after com- 
bustion. 

Much diversity of opinion still prevails as to the size 
and form of the red globules. Some recent observers 
consider them to be about one five thousandth part of 
an inch in diameter, and to be of a flattened shape, 
somewhat in the form of an almond, instead of being 
spherical, as they had formerly been supposed to be. 

The color of the blood is thought to be owing to the 
iron which it contains: this has been denied by some, 
but the notion is certainly favored by a large majority 
of the chemists of the present day. Though the ex- 
istence of iron in the globules of the blood has been 
clearly proved, it is not known in what state it exists in 
them. 



CIRCULATION OF THE BLOOD. 



Of the Serum. 

The serum is that part of the blood which remains 
fluid when spontaneous coagulation takes place. It is a 
transparent liquid, of a pale straw color, of a saline 
taste, and of a specific gravity somewhat greater than 
that of water. It has evident alkaline properties, as it 
converts vegetable blue colors to green. 

The coagulation of serum by heat is, however, its 
most remarkable property. When exposed to a tem- 
perature of 160 degrees of Fahrenheit's thermometer, 
it is converted into a white opaque solid, of great con- 
sistence. In this state, it perfectly resembles the white 
of an egg when boiled, and hence the name of albumen 
has been given to it. The serum can be coagulated 
by other substances besides heat; such as alcohol, the 
mineral acids, and tan. 

Though the whole serum appears to be converted into 
a solid substance by heat, yet if the albumen be cut into 
slices and placed in the mouth of a funnel, a few drops 
issue from it, and these have been called serosity. 

Serosity, or that part of the serum which remains 
fluid, after coagulation by heat has taken place, is found 
in small quantity only. It is obtained with so much 
difficulty separate from the serum, that it has not, till 
recently, been recognized as a distinct constituent of the 
blood. Some French chemists asserted that it contained 
jelly, though it has since been proved that it does not, 
and that this substance is not contained in the blood; it 
is still known rather by its negative than its positive 



64 CIRCULATION OF THE BLOOD. 

properties. It certainly has one property, which dis- 
tinguishes it from all the other constituents of the blood, 
and that is, its power of resisting all means that have 
hitherto been employed to coagulate it. Serosity is 
found to be much more abundant in the blood of old 
animals than in that of young ; and it forms what is 
called the red gravy in roasted beef and mutton. 

The only remaining substances that exist in the 
blood, are salts of various kinds. A thousand grains of 
serum were found to contain nine grains of salt ; six of 
these were common salt, muriate of soda, and the re- 
mainder was made up of the muriate of potash, the sub- 
carbonate of soda, and the phosphates of lime, iron, 
and magnesia. 

It appears then that the blood, when drawn from the 
body under ordinary circumstances, soon coagulates 
spontaneously; a part of it only becoming solid, and the 
rest, much the larger portion, remaining fluid. The 
solid is called crassamentum, and the fluid serum. 
The crassamentum is composed of fibrin and red glob- 
ules; the latter of which subside towards the lower 
part of the crassamentum during the process of coagu- 
lation, especially if the blood coagulates slowly. 

The serum is composed of albumen, which is coagu- 
lable by heat, mineral acids, alcohol, tan, <&c.; and se- 
rosity, which is an uncoagulable fluid, water, and 
various salts. 

All parts of the body, as was before remarked, are 
formed from the blood, however dissimilar they may 
be m appearance, structure, and properties. From it 
is secreted all the sohds as well as the fluids; and some 



CIRCULATION OP THE BLOOD. 65 

of the solids seem to possess properties very unlike 
those of the blood, as the hair, the nails and the bones. 
It is the blood that repairs the waste that is going on in 
our organs, and it also gives a stimulus to the brain 
and nervous system, without which they would be inca- 
pable of action. If the ordinary supply that is sent to 
the brain be cut off, its functions are immediately sus- 
pended; and if blood be carried there which has not 
undergone the changes, which are effected in it by the 
lungs, its functions are destroyed. What these changes 
are, will be pointed out, when treating of respiration, 
of which we shall next speak. 
6* 



(66) 



CHAPTER III. 



OF RESPIRATION. 



Respiration is that process by which air is taken 
into the lungs and expelled from them. The act by 
which the air is taken in, is called inspiration, and that 
by which it is tlirown out is called expiration. 

The wind-pipe and the lungs are, strictly speaking, 
the only respiratory organs. But as the purpose of 
respiration is to produce a remarkable change in the 
blood, it has been usual to include the blood-vessels of 
the lungs among these organs. The respiratory appara- 
tus, however, embraces all those agents which perform 
any part in the mechanical process of respiration. 

The wind-pipe is a tube composed of cartilaginous 
rings, extending from the mouth into the lungs. It is 
situated in front of the passage to the stomach, and at 
its upper extremity there is a valve, already noticed, 
which prevents the entrance of foreign substances into 
it. The rings of which it is composed, are not carti- 
laginous in their whole circumference; they are mem- 
branous in the part where the wind-pipe is joined to 
the oesophagus. 

As soon as the wind-pipe reaches the lungs, it divides 
into two branches, one going to each side; these are 



RESPIRATION. 67 

immediately subdivided into numerous smaller branches, 
which finally terminate in air cells. After a few of the 
first subdivisions, the wind-pipe ceases to be cartilagi- 
nous; all the small branches are membranous. 

The lungs occupy a large part of the cavity of the 
chest. They are divided into three parts, or lobes, on 
the right side, and two on the left. They are composed 
almost entirely of air-tubes, and air-cells, and blood- 
vessels; these, with the cellular membrane that con- 
nects them, constitute in fact, their whole substance. 
They are so vascular, that after air has once been ad- 
mitted into them, they are specifically lighter than 
water. A knowledge of this fact has led to a mode of 
determining whether infants, supposed to have been 
murdered, were born alive or not. If the lungs would 
float in water, it was decided that the children must 
have breathed, and of course have been born alive; if 
on the contrary they sunk, it was considered a proof 
that they had never breathed. 

This is, however, somewhat of a fallacious test; for 
the lungs will float in water, even though the air may 
never have been admitted into them, as soon as the 
putrefactive process has commenced. 

The lungs in the inferior animals are known by the 
popular name of lights. 

The nerves that are sent to these organs, arise in 
part from a nerve that originates in the brain, the eighth 
pair, and in part from the sympathetic nerve. 

The air-tubes and air-cells are lined by a mucous 
membrane, and the lungs are covered on the exterior 
by a serous membrane, called the pleura, which is the 



68 RESPIRATION. 

seat of the disease known by the name of pleurisy. 
This membrane not only covers the lungs on the exte- 
rior, but lines also the chest, and is constantly lubricated 
ill health, by a serous fluid which is exhaled from it. 

To give a perfectly correct notion of the process of 
respiration, it must be considered under three points of 
view; 1st, as a mechanical process; 2d, as a chemical 
one; and 3d, as a physiological or vital process. 

Under the first head must be considered the mechan- 
ism of respiration, that is the mechanical apparatus by 
which it is effected. 

Under the second will be noticed the change that is 
produced in the air by this process; and under the third 
will be considered the changes wrought in the blood, 
and the consequent effect on the whole system. 

1st. The mechanical process of respiration consists 
in inspiring and expiring atmospheric air. The appa- 
ratus by which this is accomplished is somewhat com- 
plex. The chest or thorax, as it is called, in which the 
lungs are situated, is of a conical shape, with its apex up- 
permost. The walls, or parietes of the chest, are partly 
bony and partly muscular. The breast bone forms the 
front wall, and the spine or back bone constitutes the 
posterior one. To this are attached twelve ribs on 
each side; the seven uppermost are called true ribs, 
because they unite by distinct cartilages to the breast 
bone; and the five lower ones are called false, because 
some of them unite in a common cartilage before they 
are joined to the breast bone, and the others are not 
connected with it at all. The ribs form the lateral and 
superior walls of the cavity of the thorax. The inferior 



RESPIRATION. 69 

wall is formed by the diaphragm, a muscle of great 
power which separates the cavity of the thorax from 
the cavity of the abdomen. It is attached at its cir- 
cumference to the lower part of the chest, and when in 




Trunk or the Human Skeleton-, a The ster- 
num or breast bone, b b The spine, c c c c The ribs. 

a state of relaxation, the centre is considerably elevated 
above the circumference. 

Between the ribs are situated numerous small mus- 
cles, which are called intercostals, from their situation, 
and which are supposed to elevate the ribs. 

There are other muscles which may be considered 
respiratory muscles, as they have the power of contrib- 
uting to the enlargement of the cavity of the chest. 

The lungs are in contact with the inner side of the 



70 



RESPIRATION. 



cavity of the chest. In inspiration this cavity is en- 
larged, sometimes in one direction principally, and at 
others in all. 




The Diaphragm during expiration, a Its 
tendinous centre, h b Its fleshy sides. 
c c The lateral cavities of the chest in 
which the lungs lie. 



Ordinary inspiration may be accomplished by the i 
action of the diaphragm alone. The contraction of this i 
muscle necessarily depresses its centre, which was be- ; 
fore elevated towards the lungs. The instant this takes j 



RESPIRATION. 



71 



place, the air rushes into the lungs through the wind- 
pipe, and thus prevents a vacuum, which would other- 
wise be produced between the chest and the lungs, 
In every inspiration, the ribs are somewhat raised, 




The Lungs of Man. a The heart, b b The Lungs, 
c c The Diaphragm. 



though ordinarily in a very slight and almost impercep- 
tible degree* 

In a full inspiration, there is an evident elevation of 
the ribs, and in a forced inspiration, this elevation is 
much greater ; the cavity of the chest being in both 
cases much more enlarged than in an ordinary inspira- 
tion. 

This enlargement of the chest by the elevation of 
the ribs, is owing to the oblique direction in which the 



72 RESPIRATION. 

ribs are placed ; so that when they are raised up, they 
are also turned outwards, and the cavity of the chest is 
consequently much enlarged. 

The ribs are elevated in inspiration by the contrac- 
tion of certain muscles. It was the opinion of Haller 
that this elevation was produced by the intercostal mus- 
cles, those muscles that are situated between the ribs. 
He supposed that the first rib was immoveable, and 
served as a fixed point, and that the contraction of the 
muscles that arose from it, raised the next rib, and that 
in this way all the ribs were elevated. This opinion 
has been generally adopted by physiologists till very 
recently, when it has been called in question by the 
celebrated Magendie of Paris. He denies that the 
first rib is immoveable, and appeals to every one's ex- 
perience for the correctness of his opinion, and conse- 
quently denies that the intercostals are the agents by 
which the ribs are elevated. This effect, he says, is 
produced by those muscles which arise from the head, the 
spine and the superior extremities, and which are inserted 
into the chest. His opinion as to the mode in which 
inspiration takes place, differs from the one that is gen- 
erally received, merely as to the particular muscles that 
elevate the ribs : th.e effect he admits to be the same, 
though produced by different agents. 

Expiration, by which the air is expelled from the 
chest, is merely the reverse of inspiration. It has, like 
it, three degrees, the ordinary, the full, and the forced. 
The relaxation of the inspiratory muscles, and a slight 
contraction of the expiratory ones, enable the ribs and 
the breast bone to resume their ordinary situation. In 



RESPIRATION. 73 

forced expiration, however, the cavity of the chest is 
still farther diminished, by a powerful contraction of the 
expiratory muscles. 

It is not easy to decide how much air is taken into 
the lungs at each inspiration. It is obvious that the 
quantity must be very different in different individuals, 
from the great difference that is known to exist in the 
size of the chest. The quantity too must differ in the 
different kinds of inspiration, much more being taken 
in in a forced inspiration than in an ordinary one. But, 
after allowing for these differences, physiologists have 
been unable to determine, with any degree of accuracy, 
the quantity of air taken into the lungs at each ordinary 
inspiration by an individual of the common size. The 
experiments, however, in which the most confidence is 
placed, fix the quantity at forty cubic inches for an or- 
dinary inspiration ; it has also been shown that one hun- 
dred and seventy cubic inches can be expelled from 
the lungs by a forcible expiration, and that one hundred 
and twenty cubic inches will still remain in them; so 
that the lungs may be considered as containing two 
hundred and ninety cubic inches in their quiescent 
state: to this amount must be added sixty inches for an 
ordinary inspiration, which will make the lungs contain 
three hundred and thirty inches in their distended state. 
Assuming these calculations to be correct, it follows 
that about one eighth part of the air contained in the 
lungs is changed in each inspiration. 

The number of inspirations in a minute varies some- 
what in different Individuals, the smallest number taken 
in health by an adult male is not less than fourteen, and 
7 



74 RESPIRATION. 

they rarely exceed twenty-five. Eighteen is consider- 
ed as the average number. Children and females 
breathe more rapidly than men ; but even allowing that 
eighteen inspirations are made in a minute, it makes 
the number taken in twenty-four hours amount to more 
than twenty-five thousand, and the quantity of air re- 
spired in that time exceeds one million cubic inches. 

2d. Respiration may be considered as a chemical 
process, if we regard the changes it produces in the air 
which is taken into the lungs. Atmospheric air is a 
transparent, compound fluid, elastic and compressible. 
It is composed of oxygen and azote, in the proportion 
of twenty-one parts of the former to seventy-eight of 
the latter, and one part of carbonic acid gas, or fixed 
air. Oxygen, or vital air, as it is sometimes called, 
because life cannot exist without it, though it is unfit 
for respiration unless combined with other gases, is 
somewhat heavier than atmospheric air. It takes its 
name from two Greek words, which mean the genera- 
tor of acid, because it was supposed till recently to be 
the sole acidifying principle. It is a simple substance, 
and enters freely into combination with a great variety 
of other substances. Combustion cannot take place 
without it. 

Azote, or nitrogen, is so called, because life cannot 
be supported by it. It always exists in a gaseous state; 
it is insoluble in water, and not so heavy as atmospheric 
air. A lighted candle plunged into it is immediately 
extinguished ; it forms the basis of nitric acid, enters 
largely into the composition of all animal, and some 
vegetable substances. Like oxygen, it is a simple sub- 



RESPIRATION. 75 

Stance, and forms, when combined with it in different 
proportions, compounds of very dissimilar characters ; 
such as atmosplieric air, nitric acid, and nitrous oxyd, 
or exhilarating gas. It does not possess any positive 
deleterious properties, and it destroys life, when it is 
respired alone, merely by its negative ones. 

Carbonic acid gas, or fixed air, which constitutes 
only one part in the hundred of atmospheric air, is not 
a simple substance, but is composed of carbon and oxy- 
gen. It is called an acid, because it has the property, 
though in a slight degree, which is peculiar to acids, of 
turning vegetable blues to red. It is soluble in water, 
and much heavier than atmospheric air. It is incapa- 
ble of supporting respiration or combustion. A lighted 
taper plunged into it is immediately extinguished ; and 
if an animal attempt to breathe it but for an instant, he 
is deprived of the appearance of hfe ; and unless he be 
immediately furnished with atmospheric air, he cannot 
be resuscitated. It is this gas which is found in the 
bottoms of wells and cisterns, and which renders them 
so unsafe to those who enter them without proper pre- 
caution. 

While it is certain that carbonic acid gas possesses 
deleterious properties of a positive character, that ren- 
der it unfit for respiration, even if it could be taken in- 
to the lungs, it is productive of very agreeable effects, 
when conveyed into the stomach. When dissolved in 
water, it imparts to it a pleasant, acidulated taste. It 
forms the sparkling property of mineral waters, both 
natural and artificial; and in this way it is freely drank, 
without any injurious consequences : on the contrary, 
its effects are oftentimes salutary. 



76 RESPIRATION. 

The air that we breathe, it thus appears, is a com- 
pound fluid, consisting of twenty-one parts of oxygen, 
seventy-eight parts of azote, and one part of carbonic 
acid gas. It is next to be seen what changes are pro- 
duced in this fluid by respiration. 

The ancients knew nothing of the changes effected 
in the air by the lungs. Their observ^ation, to be sure, 
taught them, that respiration was essential to life ; but 
they attributed this to the supposed effects w^hich this 
process had on the circulation of the blood. It did not 
of course escape them, that the air thrown out of the 
lungs is W'armer and more moist than when taken into 
them ; but they were not aware that any other change 
was produced in it. This is not to be considered re- 
markable, when w^e recollect, that they w^ere ignorant 
of the compound nature of atmospheric air. 

The first approach towards the truth was made by 
Mayow, an English philosopher, in the year 1674. 
He stated that the air was a compound fluid; that one 
of its constituents was essential to life and combustion, 
and that this principle combined with the blood in its 
passage through the lungs. But he went no further ; 
he knew nothing of the precise composition of atmos- 
pheric air, nor of the change which was efTected in it, 
and in the blood, by respiration. His opinions were 
received with but little favor, and the ancient doctrine 
continued to prevail for many years after his time. 

Dr. Black, in the year 1757, made known the im- 
portant fact, that carbonic acid is produced in the lungs 
by respiration ; and a few years afterwards, Dr. Priest- 
ly discovered the nature and properties of oxygen gas. 



RESPIRATION. 7^ 

He made many experiments on the subject of respira- 
tion, and though he did not perfectly explain the 
changes produced by it in the air taken into the lungs, 
his followers are indebted to his discoveries for the ad- 
ditional light which they have been able to shed on the 
subject. Many learned men have since this period la- 
bored to elucidate this obscure point in physiology ; 
and though there may be a slight difference in the re- 
sults to which they have arrived, there is sufficient 
agreement among them for all practical purposes. 

The volume of air thrown out of the lungs, is some- 
what less than that which is taken into them. It is not 
easy to estimate with precision this loss, as it varies 
under diiFerent circumstances ; the results of the ex- 
periments that have been made to settle this point, have 
diifered considerably, there being in some cases no ap- 
parent loss, while in others it was very evident. Some 
of the best physiologists of the present day, however, 
have fixed the amount lost at one eightieth part of the 
volume taken into the lungs; that is, lialf a cubic inch 
at each inspiration. 

The quantity of azote or nitrogen is nearly the same 
under ordinary circumstances, in expired air as in the 
air which is inspired. Some experimenters have re- 
cently asserted that there is a loss of azote in respira- 
tion ; but the amount of the loss is small, nor does it 
appear to be a uniform effect of respiration. It is safe, 
therefore to conclude, that if there be any loss of azote, 
it is trifling in amount, as this loss frequently has not been 

detected in very skilful and well-executed experiments. 

7# 



S RESPIRATION. 

The quantity of oxygen is diminished by respiration, 
and that of carbonic acid gas is increased. Expired 
air, instead of containing twenty-one parts of oxygen, 
hke atmospheric air, has but eighteen parts, and con- 
tains four parts of carbonic acid gas, instead of one. 
Some physiologists are of opinion that all the oxygen 
that has disappeared, may be accounted for by the car- 
bonic acid gas that is formed, while others believe that 
a portion has united with the blood. This point may 
perhaps be considered as still unsettled. We shall say 
more of it when treating of the changes produced in 
the blood by respiration. 

3d. The next point of view in which the important 
process of respiration is to be considered, is as to the 
effects which it produces on the blood that is sent to 
the lungs. It has been before explained that the blood, 
which is derived from digestion, and that w^hich is re- 
turned by the veins from all parts of the body, is car- 
ried to the right side of the heart. It is of a dark col- 
or, and unfit for the purposes of life. It is sent by the 
contraction of the right ventricle to the lungs ; it passes 
through numberless vessels of the smallest size, and is 
carried to the left side of the heart, of a bright scarlet 
color. How is this effected ^ We have seen above 
that the quantity of carbonic acid is greater in expir- 
ed, than in atmospheric air. But the oxygen contained 
in the carbonic acid gas, does not account for all the 
oxygen that is lost. Some have supposed that a por- 
tion of it unites with hydrogen, and thus forms the wa- 
tery vapor that is thrown off from the lungs. This is 
not, however, the prevailing opinion. The fact seems 
to be, that in respiration, both the air and the blood 



RESPIRATION. *79 

part with something, and receive something from each 
other. The air loses a portion of its oxygen, a part of 
which goes to the formation of the carbonic acid, and 
the remainder unites with the blood ; the blood also 
parts with some of its carbon, which unites with the 
oxygen taken into the lungs, and is then thrown out in 
the form of carbonic acid ; and another part of the 
oxygen is absorbed by the blood. Thus it appears 
that the blood parts with a portion of its carbon, and at 
the same time gains some oxygen. 

This change in the blood in respiration, has been 
called the oxygenation of the blood, by those who ex- 
plained it, on the supposition, that oxygen united with 
the blood in its passage through the lungs. It has also 
been called the decarbonization of the blood, by those, 
physiologists who believe that the change is produced 
by the discharge of carbon. The truth seems to be, 
that the blood is both oxygenated and decarbonized by 
respiration; that is, that a portion of the oxygen taken 
into the lungs unites with it, and at the same time the 
blood throws off carbon in a volatile state, which unites 
with another portion of oxygen, while the air, at the 
same time, loses some oxygen and receives some car- 
bon, and thus forms carbonic acid gas. 

It is certain, that if purple blood, out of the body, be 
exposed to the contact of oxygen gas or atmospheric 
air, it will become of a bright scarlet color. And on 
the contrary, blood of a scarlet color becomes purple 
when in contact with hydrogen, nitrogen, or carbonic 
acid gas. It seems also to be well settled, that if any 
part of the azote or nitrogen of the air be absorbed by 
the blood, it is given out again, as there is no percepti- 



80 RESPIRATION. 

ble difference between the quantity that is expired, and 
that which is inspired. It is admitted too that no hy- 
drogen is thrown out from the blood, but that the aque- 
ous vapor which is discharged from the lungs is an 
exhalation of the watery or serous part of the blood. 
There can be but little doubt, that the blood absorbs, 
during respiration, more oxygen than is necessary to 
form the carbonic acid that is expired, and that the re- 
mainder unites with the blood, and contributes to the 
remarkable change which takes place in the appearance 
and properties of this fluid in its passage through the 
lungs. 

To whatever circumstance this change may be owing, 
it is certain, that it is one essential to life. If it were 
completely suspended, even for a moment, death would 
follow. The black blood, or the blood of the veins, 
or venous blood, as it is called, cannot support the ani- 
mal functions; they require the stimulus of the red 
arterial blood. 

If respiration be suspended, the heart will for a time 
continue to throw the blood to the lungs; but when all 
the air is exhausted in these organs, so that they return 
purple blood to the left side of the heart, death imme- 
diately follows. This is owing, in great measure, to the 
circumstance that black blood is now of course thrown 
into the coronary arteries, the nourishing arteries of the 
heart; and this organ ceases to act, when not excited by 
arterial blood. The action of the brain, too, cannot be 
continued for an instant, without the stimulus of oxyge- 
nated blood, and all the organs of the body are depend- 
ent on the brain and nervous system for their power of 
action. 



RESPIRATION. 81 

Atmospheric air Is ibe only air which is capable of 
permanently supporting respiration. There are some 
others which are respirable, and others again W'hich 
cannot enter the lungs. Oxygen, w^hen respired, has 
been found highly stimulating, producing an increased 
action of the circulating system, and a consequent glow 
over the whole surface of the body. It is the opinion 
of Sir Humphrey Davy, that If It were breathed for 
any length of time, it would be productive of fatal con- 
sequences. 

Nitrous oxide, or exhilarating gas, when respired, 
produces a great excitement of the brain and nervous 
system, attended for the most part with very agreeable 
sensations. Its effects are somewhat like those of al- 
cohol, with this difference, that they are not so perma- 
nent, nor are they followed by a state of exhaustion. 

Hydrogen and nitrogen or azote, have no positive 
deleterious properties, but are injurious w^hen respired 
only by excluding oxygen from the lungs. 

Carburetted hydrogen, w^hich is the gas that is em- 
ployed for gas lights, Is positively Injurious. If it be 
respired perfectly unadulterated, it is said to produce 
instant death. When mixed with atmospheric air, it 
produces vertigo, and loss of perception. 

All the other gases are unrespirable; even carbonic 
acid gas cannot be taken Into the wind-pipe by the 
most powerful voluntary efforts. 

When it is recollected, that atmospheric air is alone 
capable of permanently supporting respiration, and that 
every adult Individual respires about one million cubic 
inches in every twenty four hours; when it is borne in 



82 RESPIRATION. 

mind how essential this process is to health and even life, 
that the functions of the body cannot be perfectly perform- 
ed, if the kings be not properly supplied, that the spirits 
are depressed, and the energies of the mind impaired, it 
must be obvious that too great care cannot be taken 
that the apartments in which we live, should be well 
ventilated, that too many individuals should not be 
crowded together, and thus be compelled to breathe 
air which has been already respired, but that the lungs 
should be constantly furnished, both by day and by 
night, with that air which can alone impart vigor to the 
physical and intellectual system. 

There are several actions of common occurrence, 
that are so intimately connected with respiration, that it 
may be proper to speak of them in this place; such as 
sighing, yawning, coughing, sneezing, laughing, and 
hiccup. 

Sighing consists in a full and long continued inspira- 
tion: its purpose, when unconnected with the state of the 
mind, seems to be to facilitate the passage of the blood 
through the vessels of the lungs. 

Yawning^ like sighing, consists of a full and pro- 
tracted inspiration; but it differs from it in being followed 
by a slow expiration, and by being attended by an in- 
voluntary distension of the jaw^s. 

Coughing is produced by a quick and powerful con- 
traction of the diaphragm, which distends the lungs with 
air, and this is driven forcibly by the contraction of 
the abdominal muscle through the trachea, for the pur- 
pose of expelling any foreign or irritating substance 
that may be lodged there or in the lungs. 



RESPIRATION. 83 

Sneezing resembles coughing; but it is more violent 
and involuntary. The irritation that produces it, is ap- 
plied to the mucous membrane of the nose, and Sir 
Charles Bell has shown that there is a connexion by 
means of nerves between this part and some of the res- 
piratory muscles. 

Laughing is the effect of an inspiration succeeded 
by short, rapid, and imperfect expirations. 

Hiccup is produced by a convulsive, rapid, and in- 
voluntary contraction of the diaphragm. In a low state 
of protracted disease, it is an alarming symptom, and 
not unfrequently precedes dissolution. There are some 
other actions that might be noticed, if it were necessary; 
but it does not seem to be, as they may be easily un- 
derstood by attending to what has preceded. The 
voice and speech will be spoken of in another place. 



I 



(84) 



CHAPTER IV. 



OF ANIMAL HEAT. 



Living animal bodies in health retain nearly the 
same degree of heat in every variety of climate, and in 
all seasons of the year. This, however, is strictly true 
only of the higher orders of animals, the warm-blooded; 
the temperature of those constituting the division, called 
cold-blooded, being much influenced by external cir- 
cumstances. The temperature of man is about 98 de- 
grees of Fahrenheit's tliermometer ; that of some other 
animals is higher, — the temperature of birds, for exam- 
ple, being about 110 degrees, it is obvious, then, that 
man lives in a medium that is for the most part colder 
than his body, the temperature of the atmosphere being 
rarely at 9S degrees, and it sometimes, in our climate, 
descends to several degrees below zero. Capt. Parry, 
with his ship's company, in their voyage of discovery, 
wintered in a climate, in which the mercury not unfre- 
quently was at 30 and 40 degrees below zero, and some- 
times even as low as 50 degrees. Yet, under these cir- 
cumstances, the bodies of warm-blooded animals have 
the powder of generating heat to such an extent, as to 
keep their own temperature within two or three degrees 



ANIMAL HEAT. 85 

of its ordinary State. Capt. Lyon, who acconipanied 
Capt. Parry in his second voyage to the Arctic Regions, 
found the temperature of an Arctic Fox on the 17th 
January, 1822, to be 106 degrees of Fahrenheit, while 
the temperature of the atmosphere was at 32 degrees 
below zero, making a difference between the temperature 
of the animal and that of the atmosphere of 138 degrees. 

On the other hand, it has been ascertained by nu- 
merous and well-conducted experiments, that the human 
body can be exposed, even for a length of time, to a 
very high temperature, without affecting to any con- 
siderable degree that of the body thus exposed. 

Dr. Fordyce and Sir Charles Blagden, in the year 
1774, entered rooms heated to some degrees above that 
of boiling water, which is 212 degrees. They, in fact, 
proved by actual experiment, that a heat of 260 degrees 
could be borne without any extreme suffering. ' In order 
to render it certain, that there was no fallacy,' says 
Sir Charles Blagden, ' in the degree of heat, shown by 
the thermometer, but that the air which we breathed, 
w^as capable of producing all the well-known effects of 
such an heat on inanimate matter, we put some eggs 
and a beef-steak upon a tin frame, placed near the 
standard thermometer, and farther distant from the 
cockle than from the w^all of the room. In about 
twenty minutes the eggs were taken out, roasted quite 
hard ; and in forty-seven minutes, the steak was not 
only dressed, but almost dry.' 

When they breathed on the thermometer, the mer- 
cury sunk several degrees, and when they expired forci- 
bly, the air felt cool as it passed through the nostrils, 



86 ANIMAL HEAT. 

though it was scorching hot, as it entered then in inspi- 
ration. 

These experiments were afterwards repeated, and it 
was found that the heat of the body soon rose to 100 
degrees, but no higher, even when exposed to a tem- 
perature considerably above 212 degrees. 

Children have been put into ovens heated to a tem- 
perature of 300 degrees, for the purpose of sweeping 
them out, without suffering any inconvenience ; and 
Chaubert, whose exhibitions under the name of the 
Fire King, excited not long since great attention, both 
in Europe and this country, is said to have entered 
them when heated to 600 degrees. 

Many other facts of a similar kind might be stated; 
but these are sufficient for our purpose : they very 
naturally lead to two important subjects of inquiry; viz. 
1st. By what organs is the animal heat generated? 2d. 
By what means is its uniformity preserved? 

1st. The Ancients had no accurate notions on the 
subject of animal heat. They believed that it was ge- 
nerated in the heart, and conveyed by the blood to all 
parts of the body. They thought that the purpose of 
respiration was to cool the blood, and thus prevent its 
heat from becoming excessive. This doctrine pre- 
vailed for many centuries: at length Mayow, whose 
discoveries respecting respiration have already been 
noticed, denied that the blood was cooled in the lungs, 
but asserted, on the contrary, that the purpose of res- 
piration was to generate heat. So little at that time 
was known of chemistry, that he was unable to give 
much appearance of probability to his views, and they 
soon passed into neglect. 



ANIMAL HEAT. 87 

Dr. Black, after his discovery of the formation of 
carbonic acid gas in the lungs by respiration, suggested 
that this process was a similar one to that of combustion, 
and that heat was generated by the formation of carbonic 
acid gas, it being extricated by the union of the con- 
stituent principles of this gas. But it was at once ob- 
jected to this theory, that, if all the heat was generated 
in the lungs, they must uniformly be at a much higher 
temperature than the other parts of the body, and this 
was known not to be the case; the difference of tem- 
perature being in fact trifling, not more than what could 
be easily accounted for on other principles. This ob- 
jection was not to be easily overcome, and, consequently, 
the theory of Dr. Black received but little attention. 

Not long after this, Dr. Crawford brought forward 
an explanation of the manner in which animal heat is 
generated, that was for a time adopted by most physi- 
ologists. He agreed in opinion with Dr. Black, that 
heat was generated in respiration, as in combustion, by 
the conversion of oxygen and carbon into carbonic acid 
gas; and the reason which he assigns for the fact that the 
lungs have not a much greater degree of heat than the 
other parts of the body, is, that the arterial blood has a 
greater capacity for caloric than the venous blood, 
and that a part of the heat formed in the lungs is ab- 
sorbed by the arterial blood, and remains in it in that 
state which is known by the name of latent heat. When 
the blood is conveyed to the various parts of the body, 
the heat is given out whenever the arterial blood is con- 
verted into venous, as it is in the various processes of 
nutrition. 



88 ANIMAL HEAT. 

The heat generated in respiration does not all go into 
the arterial blood; a portion of it is absorbed by the air 
which comes into the lungs, and another portion is taken 
up by the aqueous vapor which passes from them. 
Numerous well-conducted and ingenious experiments 
were made by Dr. Crawford, which seemed to corrob- 
orate his views, and his theory, for a time, at least, re- 
ceived the sanction of the scientific men of Europe. 

At length Mr. Brodie attempted to show that heat 
was not generated by respiration, and that its produc- 
tion in animals must be considered as a vital process, 
dependent in a great measure on the brain and nervous 
system. It was known that when the brain and the 
lungs were destroyed, either by removing the former or 
by decapitating the animal, the action of the lungs could 
be for a time maintained by artificially inflating them. It 
was also known, that during this process, the blood under- 
went the ordinary change in appearance; that is, was 
converted from a purple color to scarlet; the object of 
Mr. Brodie was to ascertain, if under these circumstances 
heat was evolved. After repeated trials, he was satis- 
fied that it was not, but that, on the contrary, if the air 
thrown into the lungs were colder than the animal, it 
reduced the temperature of the body. Hence he infer- 
red that the production of carbonic acid gas by respira- 
tion has no connexion with the generation of animal 
heat, but that this is dependent on nervous influence. 
His experiments w^ere so simple, and apparently so de- 
cisive, as to receive at once the general assent of physi- 
ologists. But his conclusions are not admitted to be 
just at the present day; nor have the same results been 



ANIMAL HEAT. 89 

obtained from his experiments when repeated by some 
eminent men. He seems to have overlooked the fact, 
that it was not maintained by Dr. Crawford's theory that 
heat was evolved in the lungs, but that, on the contrary, 
it was agreed that it w^as absorbed as fast as it was gen- 
erated by the air, by the aqueous vapor, and by the ar- 
terial blood. 

Dr. Wilson Philip supposed that the rapid cooling of 
the body, when the lungs were artificially inflated, was 
owing to the large quantity of air thrown into them; and 
in his experiments, and in those of Mr. Legallois of 
France, it was found, that when artificial respiration 
was produced by a moderate quantity of air, the 
body, though it cooled, did not cool so fast as a 
dead body in which this process was not resorted to; 
and that it parted with a greater degree of heat, while 
cooling, than dead animals did under similar circum- 
stances; a clear proof that heat was generated by artifi- 
cial respiration. 

These, and other experiments of a similar character, 
have induced most physiologists of the present day to 
adopt what is usually called, the chemical theory of the 
generation of animal heat. It must be confessed, 
however, that it is not without its difficuldes, but at the 
same time, it cannot but be acknowledged, that it is em- 
barrassed with fewer than any other that has ever been 
proposed. Though it is called a chemical theory, it is 
so only in a limited sense, the laws of chemistry being, 
as m all the other operations that go on in the living 
body, modified and controlled by the laws of vitality. 
The brain and nervous system, no doubt, perform an 
8* 



90 ANIMAL HEAT. 

important part, if not in the generation, at least in the 
evolution of animal heat. 

There is one fact worthy of notice, as it seems to 
show that there must be an intimate connexion between 
respiration and animal heat; and that is, that in those 
animals whose respiratory apparatus is the most extend- 
ed, the temperature is uniformly highest. Birds, whose 
organs of breathing extend over a large part of the 
body, and who consequently require a great quantity of 
air, have a higher degree of temperature than other ani- 
mals : it is twelve degrees above that of man ; while in 
cold-blooded animals, as fishes, whose temperature is 
not much greater than that of the medium in which 
they live, but a very small quantity of blood at any one 
time is subjected to the effects of respiration. 

2d. We come next to the second inquiry, viz. : By 
what means is the temperature of the animal body kept 
nearly uniform ? All animals ordinarily live in a medi- 
um whose temperature is lower than that of their bod- 
ies, and at the same time retain nearly an uniform de- 
gree of heat. When subjected to severe cold, their 
bodies still are kept sufficiently warm to preserve life, 
by the increased action of those causes which generate 
and evolve animal heat. These, under ordinary cir- 
cumstances, are sufficient for the purpose ; but when 
the body is debilitated by any cause, and exposed for a 
long time to a very low temperature, the vital powers 
are destroyed, and death ensues. 

The more difficult point to setde is, how animals, 
when exposed to a high temperature, are capable of re- 
sisting it, or, in other words, what means have they of 



ANIMAL HEAT. 91 

generating cold. Some recent experiments warrant the 
belief, that, when thus exposed, the temperature of the 
body is increased more than was formerly thought, be- 
ing sometimes raised ten or twelve degrees. It seems 
to be probable, also, that the sanie degree of heat is not 
generated by the body in a high temperature, as in a 
low one : it is certain that less heat is formed in the 
lungs in summer than in winter. This was very strik- 
ingly shown by Dr. Edwards, of Paris, in some ex- 
periments which he made on birds. He exposed those 
animals to a freezing mixture, for the same length of 
time, in February and July, and he found that they 
' cooled six or eight times as much in the summer as 
in the winter months.' 

But admitting that a less degree of heat is generated 
in a high than in a low temperature, this alone would 
not be sufficient to account for the power which the 
body possesses, of resisting a very great degree of 
heat. The principal cause, no doubt, is the one which 
was originally suggested by our countryman. Dr. 
Franklin, and that is, the increased exhalation which 
takes place from the lungs and the whole surface of the 
body, when it is exposed to a high temperature. The 
body is thus kept cool, on the same principle that in- 
animate bodies are cooled by evaporation. It is said 
to be a common practice in India to put bottles of 
wine, covered with wet cloths, in a current of air, even 
in the heat of the day, for the purpose of cooling it. 

But it was rendered certain by an experiment of Mr. 
Delaroche, that it was in the way first suggested by Dr. 
Franklin, that the body was enabled to resist a great 



92 ANIMAL HEAT. 

increase of temperature. He put animals into a warm 
atmosphere, saturated with moisture, so as to prevent 
in great measure the exhalation from their lungs and the 
surface of their bodies. These animals immediately- 
become heated, as if they had no power of cooling 
themselves, or resisting the temperature to which they 
were exposed. 

We can understand from this why it is that in sum- 
mer those days are most oppressive, in which the at- 
mosphere is loaded with moisture, even though the 
thermometer does not indicate so great a degree of heat 
as at manv other times. 



(93) 



CHAPTER V. 



OF SECRET I OX. 



The term secretion, in its strict sense, has the same 
meaning as that of separation ; but in physiology it is 
used to denote that process by which various substances 
are separated from the blood, either with or without 
experiencing any change during their separation. Not 
only is this process called secretion, but the same term 
is also applied to the substance thus separated. Thus 
we say, that by the process of secretion, bile is formed 
by the liver ; and we also say that bile is the secretion 
of the hver. 

The function of secretion is of great importance in 
the animal economy. By it the nutritive parts of the 
blood are separated from it, and niade to contribute to 
the nourishment and growth of the body. It is by this 
means, too, that various substances are formed, which 
are intended to accomplish very important purposes in 
the system; and it is moreover by means of secretion, 
that various noxious substances are discharged from the 
blood. 

Many of the secreted substances differ very materi- 
ally irj their composition, if we may judge by chemical 



94 SECRETION. 

analysis, or their external appearance, from the blood 
itself; and yet it was for a long time maintained, nor is 
the doctrine without its advocates at the present day, 
that the process of secretion did not change their char- 
acter, but merely separated them from the blood. 
Though it is probable that the simplest of the secreted 
substances may exist in the blood in the same state in 
which they are separated from it, it is also certain that 
no trace can be found in this fluid of some of the sub- 
stances that are formed by the process of secretion. 

Various modes of examining this subject have been 
adopted. Some physiologists have made a division, 
founded on the different nature of the secreted sub- 
stances. But to this it is objected, that we probably 
do not know all of them, or if they are known, it is 
difficult, if not impossible, so to analyze them, as to 
form, in this way, a satisfactory arrangement. The 
mere fact, that so many different divisions of these sub- 
stances have been proposed by different writers, is 
enough to show that our knowledge on this subject is 
not yet sufficiently precise for the purpose. 

The simplest division, and that v/hich on the whole 
is perhaps the best, certainly the most convenient, is 
the one that is founded on the difference of the secreto- 
ry organs. In describing these, notice can be taken of 
such of the secreted substances as may be thought 
proper. This arrangement therefore will be adopted. 

The secretory organs are of three kinds : 1st, The 
exhalent vessels ; 2d, The follicles ; and, 3d, The 
glands. 

All authors have not been agreed in calling the ex^ 



SECRETION. 95 

halent vessels secretory organs; and the function which 
they perform has been by some called exhalation, se- 
cretion being considered a more complicated process. 
But, according to the definition of secretion already 
given, it is certainly proper to consider the exhalent 
system among the ftgents of this function. 

The exhalent vessels are supposed by some to be a 
continuation of the capillaries, while others maintain, 
that the function of exhalation is performed by the ca- 
pillaries themselves. It has been before stated, that the 
arteries terminate in a set of minute vessels, called ca- 
pillaries, and from these it has been imagined that the 
exhalents are sent off. These latter vessels, which 
ai*e believed to be very abundant on the internal cavi- 
ties of the body and on the surface, have the power of 
separating various substances from the blood : at any 
rate, we know that this process is constantly going on 
there, and it is of but little consequence whether 
it be performed by the capillaries, or another set of 
vessels. The exhalations have been divided into two 
kinds, the internal and the external. 

The internal exhalations are those which take place 
within the body. The serous membrane, which cov- 
ers the organs within the head, the chest and the abdo- 
men, is continually, during life, exhaling serous fluid, 
which has a close resemblance to the serum of the 
blood. This fluid is thrown out by this membrane to 
keep the surfaces on which it is exhaled, in a moist 
state, and thus to enable the organs to move easily on 
each other. 

The cellular texture also exhales a serous fluid, and 



96 SECRETION. 

in some forms of disease, this becomes excessive, pro- 
ducing a general dropsy. 

The fat is also thrown out by the process of exhala- 
tion, from the cellular texture. It is contained in cells, 
and it is supposed to be in a fluid state, when it is first 
exhaled. Its principal use in health seems to be from 
its physical properties, forming a sort of cushion under 
the skin, which protects it from injury in walking, 
standing, and other positions of the body. 

During sickness, the fat, or adipose substance, as it 
is called, is supposed to aid in nourishing the body, 
being taken up, for this purpose, by the absorbent ves- 
sels, and conveyed into the blood. It is considered an 
unfavorable symptom in disease, if emaciation does not 
take place, because it shows a w^ant of power, in the ab- 
sorbing system, which is among the last to be aifected. 
Hybernating animals, those that become torpid during 
the winter, are uniformly found to be much emaciated 
on the return of spring, they having been nourished 
during their hybernation, by the absorption of their fat. 

The fat is usually of a yellow color, and inodorous. 

The hgaments which surround the joints, are hned 
by the serous membrane, which secretes a fluid called 
synovia. This fluid is designed to facilitate the move- 
ments of the joints, by lubricating them, and thus 
enabling them to move with ease. It has been sup- 
posed by some, that this synovial membrane is not pre- 
cisely like the serous membranes: the difference, if 
there be any, is so slight, that it is not easily detected. 

In the cavities of the long bones, there is found a 
substance somewhat resembling fat, which is known by 



SECRETION. 97 

the name of marrow or medullary substance. It is se- 
creted by a delicate membrane, that lines the cavities 
of the bones. The use of the marrow does not appear 
to be known. It is not formed for the purpose of ren- 
dering the bones less brittle, as has been supposed by 
some, because it is most abundant in the bones of old 
persons, which are much more brittle than those of 
young ones. 

These are the most important of the internal exhala- 
tions. 

The external exhalations that are next to be noticed, 
are the cutaneous and the pulmonary; the one taking 
place from the skin, and the other from the mucous 
membrane that lines the internal parts of the lungs, 
wind-pipe, fauces, and mouth. 

The cutaneous exhalation or transpiration, as it is 
called, is of two kinds, one of which is insensible, and 
the other sensible; the first is called insensible transpi- 
ration^ and the second sweat. The fluid that is thrown 
off in this way from the skin, consists chiefly of water, 
with a small quantity of acid, and salts of different kinds 
with a minute portion of animal matter. 

Many attempts have been made to ascertain the quan- 
tity of insensible transpiration, that is thrown off in a 
given time. Sanctorius followed up a series of expe- 
riments on the subject for thirty years, but the results 
at which he arrived have never been considered satis- 
factory. 

Lavoisier and Seguin have investigated the subject with 
great skill, and no doubt is entertained of the general 
accuracy of their conclusions, though in investigations 
9 



98 SECRETION. 

of this kind, it is difficult to arrive at anything more than 
approximations. The following are among the results 
of their inquiries. 

1st. The greatest quantity of insensible transpiration, 
including that from the lungs as well as the skin, given 
off in a minute, is thirty-two grains. 

2d. The least quantity was eleven grains. 

3d. The average, eighteen grains; eleven from the 
skin, and seven from the lungs. 

4th. During the period of digestion, a less quantity 
is thrown off than at any other time in health. 

The uses of the cutaneous transpiration are impor- 
tant; it renders the skin more pliable, and the sense of 
touch more delicate. It is also, as we have seen before, 
the most important agent in cooling the body, when it 
is exposed to a high temperature, and there is reason to 
believe, that it carries out of the body some noxious 
property of the blood. It is certain that when it is 
lessened, or altogether stopped, the health becomes im- 
paired, and many diseases are removed when copious 
perspiration is produced. 

The pulmonary transpiration very closely resembles 
that of the skin. This aqueous exhalation, that is 
thrown off by the lungs during respiration, was at one 
time supposed to be formed in the lungs by the union 
of hydrogen from the blood, and oxygen from the in- 
spired air, thus forming water. This is not, however, 
the opinion at the present day. It is now believed, 
that this fluid is either given off directly by the blood, 
in the same way as the cutaneous transpiration, or that 
it is the watery part of the mucus, that is secreted by 
the mucous membrane of the lungs and air passages. 



SECRETION. 99 

All the internal cavities that open externally, are lined 
by a mucous membrane, and this membrane constantly 
secretes a fluid which is known by the name of mucus, 
and which has a close affinity to the transpiration from 
the skin. Some are of opinion, that the pulmonaiy 
transpiration is merely the watery part of the mucus, 
while others maintain that the watery exhalation takes 
place in addition to the secretion of mucus. 

2d. The Follicles. The follicles are very small 
sacs or bags, which are found only in the skin and 
the mucous membranes. They are therefore of two 
kinds, the cutaneous and the mucous. The pores that 
are seen on almost every part of the skin, are the out- 
lets of the cutaneous follicles. They secrete an oily 
unctuous substance, which mixes with the transpiration, 
and lubricates and softens the skin. When it does not 
pass off readily, the pores become obstructed, and appear 
as if they were filled with small worms. There is a 
small folhcle at the root of each hair. The wax in the 
passage of the ear is secreted by the folhcles. 

Every part of the mucous membrane is more or less 
abundantly supplied with follicles. They secrete a 
fluid, which is supposed to resemble very closely 
mucus; but this cannot be known with certainty, as it 
is impossible to obtain them separate. Some of these 
follicles exist on the tongue, and the small openings 
that are seen on the tonsils or almonds of the ears, 
which are situated in the back of the throat, are sup- 
posed to be the outlets for the secreted fluids of these 
small organs. 

3d. Of the Glands. Glands are bodies of various 
size, more or less of a rounded form, situated in various 



100 SECRETION. 

parts of the body. They secrete substances of very 
different character, and, in some instances, having little 
or no resemblance to the blood from which they se- 
crete them. In the more perfect glands there is found 
one or more arteries, which carry the blood to. this 
organ, a set of veins, which return a portion of it, and 
another set of vessels, which usually unite in a common 
trunk called an excretory duct, which conveys the fluid 
formed by the gland to its place of destination. The 
intimate nature of the glandular structure is wholly un- 




A Gland with an excretory duct, a a The sub- 
stance of i he gland irregular and lobulaled. bb 
The small branches, by which the excretory duct 
arises from the gland, c 'i he trunk of the excre- 
tory duct fully formed. 

known, and it is by no means determined whether the 
secretion is performed by this structure, or whether it 
is dependent on the action of the vessels alone. In 
some instances, where the secreted fluid differs very 
materially from the blood, the secretory apparatus is 
simple; while in other cases, in which it bears a close 
resemblance to the fluid from which it is separated, the 
organ by which it is effected is of a complicated char- 



SECRETION. 101 

lacter. It is probably impossible to settle a question of 
this kind by actual experiment. 

The glands, as has been observed, are of very various 
size. The lachrymal gland, which secretes the tears, 
is a flat body, not larger than a small almond, while the 
liver, by w^iich the bile is formed, weighs from two to 
three pounds in an adult in health, and sometimes, in 
disease, weighs fifteen or twenty pounds. The liver is 
supposed to differ also from the other glands in this 
respect, that its secretion is not made from the arterial 
blood. This is by no means a settled point, as some 
contend that its secretion takes place as in the other 
glands, while others are of opinion that the bile is prin- 
cipally formed from the blood of the veins. It is cer- 
tain that the liver has an artery, called the hepatic artery, 
a set of veins to return the blood to the heart, and an 
excretory duct, to convey the secreted fluid into the 
intestines: in addition to these, a very large vein, formed 
by the union of several of the most important veins com- 
ing from the abdominal viscera, and called the vena 
portse, enters into it, and is distributed throughout its 
substance. The blood in this vein is necessarily of a 
very impure chai^acter; it has parted with its nutritive 
and vital principles, and it is about to be returned to 
the heart, to be again sent to the lungs, to undergo the 
essential change which respiration effects in it. In this 
state it may be supposed to be loaded with noxious 
principles, and that it is sent to the liver for the pur- 
pose of parting w^ith some of them. This is the view 
that is taken by some physiologists; while Bichat and 
others maintain, that the bile is formed from the arterial 



102 SECRETION. 

blood, and a third class, at the head of which stands Ma- 
gendie, are of opinion that it is secreted from both the 
arterial and the venous blood. 

There are then but three known kinds of secretory 
organs: by the first the secretion is thrown out directly 
from the blood-vessels; by the second, it is formed by 
the intervention of a small sac or vesicle, called a folli- 
cle; and by the third, it is effected by a much more 
comphcated apparatus known by the name of glands. 

It is by no means easy to explain the manner in 
which secretion is performed. The same fluid is car- 
ried to the different secretory organs, and they sep- 
arate from it substances that not only differ in some 
instances very essentially from the blood, but which 
also have litde or no resemblance to each other. In 
fact, the elementary principles of some of the secretions 
cannot be detected in the blood, and their formation is 
wholly inexplicable in the present state of our knowledge. 

Various theories have been proposed at different pe- 
riods on this subject. One of the earliest attributed 
secretion to a kind of fermentation that took place in 
the secretory organs. There is, however, no evidence 
that any such fermentation does take place; nor is there 
any reason to believe, that if it does, it would be capa- 
ble of producing the effects which have been attributed 
to it. It is probable, that the advocates of this theory 
used the term fermentation, in a sense somewhat differ- 
ent from the ordinary one; but even admitting this, it 
does not remove the difficulty. 

The hypothesis that succeeded to this was a mechanical 
one, which supposed that the secreted substances already 



SECRETION. 103 

existed in the blood, witbout explaining how they got 
there, and that the secretory apparatus served as a kind 
of sieve or filter, through which only its peculiar secre- 
tion could pass, — the rest of the blood returning by the 
veins. A sufficient objection to this is, that it assumes 
what no one now believes, that the various secreted 
substances exist already formed in the blood. 

Some experiments have been made to show that se- 
cretion was dependent on nervous influence. The di- 
vision of the par vagum, a nerve that goes from the 
brain to the stomach, stops the secretion of the gastric 
liquor, and it is well known that some affections of the 
mind have a power over the secretions. Grief often, 
times produces an increased action of the lachrymal 
gland, and the thought of savory food, causes an in- 
creased flow of saliva. Though the nerves probably 
have an influence in the higher orders of animals over 
the secretory organs, yet there are some animals in whom 
no nervous system can be discovered, and yet in them 
secretions go on. 

The doctrine which is received with the greatest fa- 
vor at the present day, is what may be called the 
chemical theory of secretion. According to this hy- 
pothesis, the secreted substances do not exist in that 
form in the blood, the elements of them only being 
there; and it is by the combination of these elements in 
various proportions, that all the secretions of the sys- 
tem are produced. There are no doubt some well- 
known facts in chemistry, that give a plausibility to 
this view of the subject. It is certain, that a few ele- 
ments, united in different proportions, are capable of 



104 SECRETION. 

producing compounds of very different characters. But 
giving to chemistry all the power that its warmest ad- 
vocates can claim for it, it must still be admitted, that 
there is a controlHng power, which is vital agency. If 
it were not so, all the secretions could be made out of 
the body as perfectly as they are formed in it ; and if 
the blood, which is carried to the different secretory 
organs,, was acted upon only by chemical laws, why 
would not the results be the same in every instance ? 
Or, in other words, what should produce a difference 
in the secretions, if they are the effect only of the com- 
bination of different elements of the blood in various 
proportions under the influence of chemical laws alone.'^ 
But even admitting the influence of vitality, the subject 
is not without its difficulties, for we find that some sub- 
stances are secreted, the elements of which have never 
been found in the blood, or, if found at all, only in very 
minute quantity. 



Of some of the Secreted Fluids. 

It may be proper to say a few words of the nature 
and chemical composition, as far as they are known, of 
some of these fluids. 

Of the Saliva. 

This is formed, as has been already stated, by three 
pairs of glands, situated in and about the mouth. It is 
poured out very copiously at all times, but more espe- 
cially while eating. It is doubtful whether it contributes 



.SECRETION. 105 

in any degree to the process of digestion : its principal 
use in this process seems to be to prepare the food, by- 
mixing with it, so that it can pass with ease into the 
stomach. It assists also the voice, by keeping the 
mouth in a moist state. 

Pure saliva has never been subjected to analysis, be- 
cause it is nearly impossible to obtain this fluid, unless 
mixed with the other fluids of the mouth. It is no doubt 
true, however, that the salivaconstitutes the greater part 
of the fluid thus obtained, so that the result of any chem- 
ical examination of it may be considered sufficiently ac- 
curate for all practical purposes. The saliva is a mild, 
viscid fluid, without smell, taste or color, and a little 
heavier than water. One thousand parts of it contain 
nine hundred and ninety-two parts of water, and the re- 
maining eight parts are made up of various kinds of 
salts, with a small quantity of mucus, and a minute por- 
tion of animal matter of a peculiar character. The 
properties of the saliva are not always the same : it 
sometimes contains acid, and it is well known to be 
acrid after long fasting. It is on this account that it is 
a popular remedy in that state for ring-worms, and 
some other cutaneous affections. 



Of the Pancreatic Fluid. 

The pancreas, or sweet-bread, is a gland situated be- 
hind the stomach ; and its secretion, as was observed 
when treating of digestion, is poured into the first in- 
testine. From the resemblance, both in structure and 
appearance of this gland, to the salivary glands, it has 



106 SECRETION. 

been thought to serve the same purposes. The pan- 
creas secretes but a small quantity of fluid, and it is not 
easy to obtain much of it for the purposes of examina- 
tion. Mr. Magendie, who has probably made the most 
experiments on the subject, states it to be of a yellow- 
ish color, saltish taste, inodorous, possessing alkaline 
properties, and capable of being coagulated by heat. 
He further adds, that the secretion of it is not increas- 
ed during digestion, but rather diminished. The uses 
of this fluid are not known ; it does not resemble the 
sahva in its properties, so much as has been thought. 



Of the Gastric Juice. 

It is remarkable that this fluid, which is known to 
possess properties of such activity, should so closely 
resemble the saliva, as it seems to do. Chemical anal- 
ysis has not been able to detect in it anything which 
can account for its power of dissolving the different 
substances that are put into the stomach. Dr. Prout 
has recently ascertained, that during the process of di- 
gestion, muriatic acid is present in the stomach ; it is 
this acid, which by its combination with soda, forms 
common salt. It is not supposed, however, that the 
acid exists in the gastric juice, but that it is in some 
way evolved from the food, and then becomes an im- 
portant agent in converting it into chyme. The pre- 
vailing opinion is, that the gastric fluid has neither acid 
nor alkaline properties. It is remarkable not only for 
its solvent power, but for two others. The first of 
these is the power of coagulating albuminous fluids, 



SECRETION. 107 

and on this depends the process of making cheese, as 
has been ah'eady stated. The second is the power of 
preventing putrefaction, and of even removing in some 
degree the effects of it when it has already commenc- 
ed. Animals, that live on carrion, often take their 
food in a half putrid state, and it has been found that 
the first operation of the stomach is to remove the fee- 
tor from it. Experiments made with the gastric juice 
out of the body, show conclusively that it has, to a cer- 
tain extent, the property of preventing putrefaction. 
Notwithstanding it is possessed of such wonderful prop- 
erties, yet, on analysis, nothing can be detected in It 
essentially different from the constituent principles of 
that mild. Inert fluid, the saliva. 



Of the Bile. 

This fluid, as is well known, is secreted by the liver, 
the largest gland in the body. It differs very material- 
ly, both In appearance and properties, from the blood. 
It Is of a greenish, yellow color, of an excessively bit- 
ter taste, thick, sometimes transparent, and sometimes 
opake. It contains water, albumen, resin, and a great 
variety of salts. The purposes of the secretion of the 
liver appear to be two-fold ; the bile probably assists 
in some way, which is not precisely known. In the for- 
mation of chyle, and there is reason to believe that it Is 
made the vehicle to carry out of the system, under 
certain circumstances, substances, which. If retained, 
might prove noxious. 



108 SECRETION. 



Of the Tears. 

The tears, which are secreted by the lachrymal 
glands, consist principally of water. They are limpid, 
inodorous, and of a saltish taste. They contain minute 
quantities of common salt, phosphate of soda, soda, 
lime, and mucus. 



■ OmTip i mD Lm; J m 



(109) 



CHAPTER VI. 



OF ABSORPTION. 



Absorption is that process by which the substances 
that are designed to contribute to the nourishment and 
growth of the body, are conveyed into the blood. 
There is another office which is performed by another 
part of the absorbent system. The particles of which 
the body is composed, are constantly changing ; they 
lose their power of performing their functions; they 
are then removed by a distinct set of absorbent vessels, 
and carried into the veins, and new ones are brought to 
supply their place. There are then two sets of ab- 
sorbent vessels; one of them, connected with the diges- 
tive function, is situated in the abdomen ; the other is 
spread throughout the body. The vessels of the first 
are called lacteals, and those of the second lymphatics. 
Each of these will be described separately; and after 
doing this, we shall then examine the question, whether 
absorption is performed by any other vessels. 

The lacteals, as has been before said, are so called, 
because the fluid they contain, resembles milk in ap- 
pearance. They take up the chyle from the small in- 
testines, and convey it to the thoracic duct. This part 
of the absorbent system may be said to consist of lac- 
10 



110 ABSORPTION. 

teals, mesenteric glands, and the thoracic duct ; this 
last, however, is connected both with the lymphatics 
and the lacteals. 

The lacteals originate in the inner coat of the small 
intestines, but in what manner is not precisely known. 
Some have undertaken to describe the mouths of the 
lacteal vessels which take up the chyle ; but most 
anatomists have been unable to discover them. These 
vessels are very minute at their origin, and of a very 
delicate structure, in their whole course. As they 
go from the intestines, they pass along the mesen- 
tery, which is a very thin membrane, formed by a du- 
plicature of the serous coat of the intestines ; in passing 
through the mesentery, they are seen to increase in size, 
very much in the same manner as the veins do, which 
begin in numerous small branches, that soon run into, 
and are connected with others, fewer in number, but 
of larger size. The lacteals are furnished with a large 
number of valves, which are of sufficient power, under 
ordinary circumstances, to prevent the retrograde mo- 
tion of the chyle. As they pass along the mesentery, 
they are connected with numerous small bodies, known 
by the name of the mesenteric glands. The use of 
these is unknown. The various branches of the lac- 
teals soon unite into three or four trunks, which pour 
their contents into the thoracic duct. 

This vessel is situated near the spine, and it differs 
from the other absorbent vessels chiefly in its size, be- 
ing much the largest of any. Its coats, though thin and 
transparent, are strong and elastic, and, like the other 
absorbents, it is furnished with many valves. At its 



ABSORPTION. Ill 

commencement, there is a sort of reservoir, which is 
called the receptacle of the chyle, (receptaculum chyli.) 
The thoracic duct usually consists of a single trunk ; 
sometimes, however, there are two of nearly equal size; 
while in other instances, there has been found one of 
the ordinary size, and one or two smaller ones. It 
passes up along the spine, from the place where it origi- 
nates, which is about the fourth dorsal vertebra, to half 
an inch above the subclavian vein ; it then bends down 
and pours its contents into this vessel. 

The other set of absorbent vessels are called lym- 
phatics. They received this name from the fluid they 
contain, being called lymph. They are very numer- 
ous, probably in all parts of the body, though they have 
not been discovered in the eye or the brain. Their 
structure resembles that of the lacteals. 

They may be divided, from their situation, into two 
classes, the superficial, and the deep-seated. They 
are connected hke the lacteals, with glands, which are 
called lymphatic glands. These small bodies are more 
numerous in some parts of the body than in others ; 
several of them may be found in the groin, under the 
arm and in the neck. From exposure to cold and oth- 
er causes of irritation, they frequently inflame and en- 
large, and are vulgarly known by the name of kernels. 
They often become the seat of disease, particularly of 
the scrofula, or king's evil. The lymphatic vessels of 
the extremities frequently inflame from slight causes. 
A trifling wound of the finger will oftentimes be fol- 
lowed by inflammation of some of the lymphatics of 
the arm, and these little vessels may in that state be 



112 ABSORPTION. 

traced by their red color up to the arm-pit, in which 
some of the small glands will be found enlarged, sore, 
and perhaps painful. 

The lymphatic vessels, in their course towards the 
thoracic duct, become fewer in number, and somewhat 
larger, though they never attain any considerable size. 
They pursue a similar course with the lacteals, and part 
of them finally pour their contents into the thoracic 
duct, and the remainder terminate in a trunk on the 
right side, about as large as the thoracic duct, though 
much shorter. The absorbents were not known as a 
distinct system, by the ancients. The lacteals were 
discovered by Aselli, in 1622, and the lymphatics 
about thirty years afterwards. A complete knowledge 
of the absorbent system was not obtained at once ; and 
the fact of the termination of the lacteals in the thoracic 
duct, was not known till some years after the discovery 
of these vessels. 

The lacteals and lymphatics, though they resemble 
each other very closely in their structure and termina- 
tion, differ as to the nature of the fluids which they 
convey, and also as to tlus purposes of their functions. 
The lacteals open in the small intestines, and take up 
the chyle. The mouths by which this fluid enters, 
have never been discovered, though the contrary has 
been maintained. These mouths seem to be furnished 
with the power of selecting from the mass that is pass- 
ing through the intestines, the fluid which is to be con- 
veyed into the blood, and become a part of it. The 
lacteals, therefore, have the power of taking up the 
chyle only; or perhaps it would be more proper to say, 



ABSORPTION. 



113 



that they are endowed with the power of rejecting eve- 
ry thing that is presented to them but the chyle ; while 
the lymphatics not only take up all the various constitu- 
ents of the body, both fluid and solid, when they have 




Ji ^\aSi piece of a small intestine, hhhh are tlie superficial lacleals. 
ccc is the mesentery, a delicate, but firm membrane, consisting of 
two layers, by which the intestines are connceted with the Sj)ine, and 
■within the folds of which the deep-seated lactcais pass, d d d and 
tee the two sets of absorbent glands //the receptacle of the chyle. 
^the thoracic duct, i i the lymphatics, coming from difierent paits 
of the body. A the aorta, the great artery. 

ceased to be of use to the parts to which they belong- 
ed, and convey them into the blood, either to be thrown 
out of the body, or to be rendered again fit for the 
purposes of life, but they also absorb foreign and extra- 
neous substances, when presented to their mouths. 
10* 



114 ABSORPTION. 

Medicines will act as powerfully, when introduced into 
the system through the absorbents, as when they are 
given by the stomach. 

The ordinary functions of the body in health require 
incessant action of the absoibents : the circulating sys- 
tem, with its myriad of minute vessels, is constantly 
depositing new particles, and the old ones are taken 
away by the absorbent system. Though the vessels 
of this system have not yet been traced in all the ani- 
mal organs, there can be no doubt that they are all fur- 
nished with them. No part of the body is exempt from 
this process of composition and decomposition, the for- 
mer being effected by the vessels of the circulating sys- 
tem, and the latter by those of the absorbent system. 

Some of the uses of the lacteals and lymphatics seem 
to be well understood ; the former convey the chyle, 
the product of digestion, into the mass of the circula- 
tion, and the latter carry another fluid, somewhat re- 
sembling the chyle, called lymph, which is collected 
from all parts of the body. The uniformity in the 
character of the lymph, has led some physiologists to 
doubt whether the lymphatics take up all the extrane- 
ous bodies that are known to be removed by the ab- 
sorbent system. 

The nature of the lymph is not yet well understood. 
The term has been applied to a variety of fluids, but it 
is now restricted so as to denote that only which is con- 
tained in the lymphatic vessels. Mr. Magendie denies 
the common doctrine, that the lymph originates from 
the decomposition of the component parts of the body, 
but maintains it to be a certain part of the blood, which 



ABSORPTION. 115 

instead of returning by the veins, is carried back to the 
heart by the lym})hatics and the thoracic duct. He 
rests his opinion principally on the uniformity of the 
character of the lymph. It is foreign to our purpose to 
enter into a discussion of this subject; it maybe proper 
however to remark, that this does not seem to be a suf- 
ficient reason for abandoning the received notion, which 
considers the absorbents as the principal agents, which 
remove the various parts of the body when they no 
longer perform the functions for which they w^re in- 
tended. 

The lymph, when examined out of the body, is usually 
of a reddish color and of a saline taste. It soon coagu- 
lates and becomes of a deeper color. This coagulated 
lymph consists of two parts, one of which is solid, with 
numerous cells, and these cells are filled with fluid. A 
thousand parts of lymph contain nine hundred and twen- 
ty-six parts of water, sixty-one of albumen, and the 
remaining thirteen parts are various kinds of salts. 

The office of the absorbent glands, the mesenteric 
and lymphatic, is not known. It is presumed to be im- 
portant, because every absorbent vessel in its course 
passes through one of them ; but it is also true, that 
they are found only in the higher order of animals. 
Conjecture has assigned to them two uses; the one is 
to assimilate the fluid which is passing through the ab- 
sorbent system to the blood, with which it is soon to 
be mixed; and the other is to resist the introduction 
into the circulation, of foreign noxious substances. 

It was known long before the discovery of the lac- 
teals and lymphatics; in fact, even in the time of Galen, 



116 ABSORPTION. 

who lived a little more than a century after the birth of 
our Saviour, that absorption took place, and this office 
was supposed to be performed by the veins. 

For many years after the discovery of the absorbent 
vessels, it was thought that they were not the exclusive 
agents of absorption, but that the venous system also 
performed a part. This was the opinion of Haller, 
Boerhaave, and other distinguished physiologists, and 
was in fact the prevailing one till about the middle of 
the last century. Though the veins had never been 
proved to be the agents of absorption by direct experi- 
ment, it was still asserted that it had been ascertained, 
by injections, that there was a free conmiunication be- 
tween the venous and the absorbent systems, proving 
an identity in their functions; it was also known that ab- 
sorption took place in parts where no absorbents could 
be detected, and it was thence inferred, that it was accom- 
plished by the veins. Dr. William Hunter, of London, 
and Dr. A. Monro, sen. undertook a series of experi- 
ments about the same time, which induced them to be- 
lieve that absorption was performed exclusively by the 
lacteal and lymphatic vessels. They maintained that 
in those cases where a communication was discovered 
by means of injections between the absorbent and venous 
systems, it was the effect of accident, owing to the 
rupture of one or more vessels in consequence of the 
force with which the injection was thrown in. They 
extended their researches also to those parts, which had 
before been supposed to be destitute of absorbents, and 
in some of these they discovered these vessels, which 
they thought authorized them to infer that they existed 



ABSORPTION. 117 

in all, though they were in many parts of so delicate a 
structure, as to elude discovery* 

Mr. John Hunter made some experiments on the 
subject about this time, that seemed to remove all doubt 
upon the subject, and physiologists became agreed in 
rejecting the doctrine of absorption by the veins. He 
opened the abdomen of a living dog, removed from a 
portion of the intestine all its contents; he emptied the 
veins of their blood by puncturing them; he tied the ar- 
teries that went to the part, and thus prevented blood 
from being carried there; he then injected into the in- 
testine warm milk, and kept it there by placing ligatures 
above and below, and then returned the intestine into 
the abdomen. In half an hour he drew it out ; on ex- 
amination he found that the veins were as nearly empty 
as before, that they contained no whii^ fluid, while the 
lacteals were full of it. From this he concluded that 
the lacteals absorb, and that the veins do not. 

For a time there seemed to be no subject in physi- 
ology on which there was less diversity of opinion than on 
that of absorption; the only question appeared to be to 
whom belonged the credit of having first established the 
fact that the lacteals and lymphatics were the exclusive 
absorbent vessels. But recently this opinion has been 
controverted by Mr. Magendie, of Paris, one of the 
most distinguished physiologists of the age, who has 
supported his views by so many ingenious and well- 
conducted experiments, as to satisfy most persons of 
their correctness. He repeated the experiments of Mr. 
Hunter, but obtained different results, The same thing 
has happened to others. From numerous experiments, 



118 ABSORPTION. 

he came to the conclusion, that the lacteals absorb the 
chyle only, but do not take up the various liquids that 
are taken into the stomach, and which are known to 
pass unchanged into the blood. If < iluted alcohol be 
given to a dog while he is digesting solid aliment, the 
chyle, on examination, in half an hour, will not be found 
to have the odor of alcohol, though it will be very 
perceptible in the blood, and alcohol can be obtained 
from it by distillation. Water and other drinks which 
are not converted into chyle, he believes are taken up 
and carried into the circulation by the veins of the in- 
testines. 

Having established, by numerous experiments, that 
absorption by the veins takes place in the intestines, 
Mr. Magendie instituted others, to show that the veins 
have this power in other parts of the body. It is un- 
necessary to relate many of these: there is one, how- 
ever, of so decisive a character, as in great measure to 
establish his views, if nothing else could be adduced in 
support of them. He divided all the parts of the hind 
leg of a dog, except the artery and the vein; a poison- 
ous substance was then applied to the foot, — in four 
minutes its effects were visible, and in ten minutes the 
animnl cied. It seems hardly possible to suppose that 
the poison was introduced into the system in this 
experiment, except throught he vein; but to avoid all 
possibility of error, he made another. He introduced 
small leaden tubes into the artery and vein, secured 
them there by ligatures, and then divided the vessels, so 
that all the blood must pass through these tubes in 
going from the extremity to the body of the anirnal; yet 



ABSORPTION; 119 

the poison, applied in the same way, produced the same 
result as in the former experiment. 

On the whole, then, it may be said with regard to 
absorption, that it is performed by the lacte , the 
lymphatics, and the veins. The former, under ordinary 
circumstances, take up only the chyle, which is destined 
to the nutrition of the body. The lymphatics carry a 
thin semi-transparent fluid, known by the name of 
lymph, which may sometimes serve the purpose of nu- 
trition, but which it probably does not do, while the 
lacteals are able to perform their functions. The veins 
absorb from the intestines liquids of various kinds that 
have been taken into the stomach, and which have not 
been converted into chyle, and in other parts of the 
body, they perform the ordinary ofHce of lymphatics. 
If we acknit the absorption by the veins, it will perhaps 
satisfactorily account for the fact, that the venous sys- 
tem has so much greater capacity than the arterial ; for 
if it had no other oflice to perform, than to return the 
blood that had been distributed to the various parts of 
the body, it would be reasonable to suppose, that the 
capacity of the veins would be less than that of the ar- 
teries; but it is known to be greater, and if it has the 
function of absorption to perform, there would seem to 
be a reason for it. 

Whatever be the agents of absorption, it is a process 
that goes on unceasingly in health and disease. In the 
former, it conveys the nutritive part of the food into the 
blood, and carries into the s-^me fluid the several con- 
stituents of the different organs of the body, when they 
are no longer able to perform their original functions. 



120 ABSORPTION. 

In some forms of disease, its action is more obvious. 
Water, that is poured out in the different kinds of 
dropsy, is often absorbed with great rapidity; and it is by 
absorption, that nature often removes hard and sohd tu- 
mors from various parts of the body. Perhaps the 
effect is no where more distinctly seen than in the 
neighborhood of an aneurism. An aneurism is an en- 
largement of an artery, one of the vessels which distrib- 
utes the blood throughout the body. From accident 
or disease, one of the coats of this vessel is w eakened 
or destroyed ; a swelling forms at the place, and it soon 
becomes a pulsating tumor, and if it be in a large ar- 
tery, it beats with great force. This disease frequently 
occurs in the aorta, the largest artery that goes off from 
the heart. A large tumor is formed in the chest; it 
presses against the ribs and breast-bone, and finally they 
are absorbed. Now the absorption of these bones is 
not produced by the aneurism; it is a process that is 
constantly going on, but it is the aneurism which pre- 
vents the deposition of the new particles of bone, which, 
under ordinary circumstances, would have been depo- 
sited to supply the place of those taken up by the ab- 
sorbents. 

There has been much diversity of opinion on the 
question of cutaneous absorption; some maintaining that 
the skin absorbs, while others deny it, and both opinions 
being supported by numerous experiments. The point 
is not yet settled; it is probable, as it oftentimes hap- 
pens, that the truth lies between the two parties. There 
is reason to belie vj that the skin does absorb in a 



ABSORPTION. 121 

moderate degree, in some parts of the body, though it has 
not the power to the extent which has been claimed 
for it. 

Those who have denied cutaneous absorption have 
asserted, that the error arose from the vapor of the sub- 
stance employed in the experiment being allowed to en- 
ter the lungs, and that if pains were taken to guard against 
this, no absorption took place. In other experiments, 
this has been carefully guarded against, and distinct 
traces of the substance used in the experiment have 
been found in some of the secretions. 

It is difficult to explain the mode in which the ab- 
sorbents act, or, in other words, to show^ in what manner 
they take up and circulate the fluids which pass through 
them. It has been supposed by some, that these fluids 
are drawn into the mouths of the absorbents, by a kind 
of capillary attraction, while others have thought that 
it is the result of a vital law that cannot be explained 
on mechanical principles. There can be no doubt that 
the lacteals have a power of selecting such parts of the 
contents of the intestines, as can be rendered tributary 
to the purposes of nutrition, and rejecting such as could 
not be assimilated with the blood. But if it be admit- 
ted that the fluids are taken into the mouths of the ab- 
sorbent vessels by a kind of capillary attraction, it will 
not explain how the fluids are carried along these ves- 
sels. The coats of the absorbents are remarkably thin 
and delicate, and destitute of muscular fibres. They 
have not, therefore, the contractile power of the capil- 
lary vessels, and consequently cannot themselves pro- 
pel the fluids which they contain. Neither is there 
11 



122 ABSORPTION. 

connected with the absorbent system, a powerful mus- 
cular organ, like the heart, which, by its contraction, 
can throw a fluid through the system of vessels with 
which it is connected. We must therefore regard the 
course of the fluids through the absorbent system, as 
one of those operations, that are dependent on vital 
laws, and not to be explained on the common princi- 
ples of mechanics. 



(123) 



CHAPTER VII. 



OF NUTRITION. 



The process by which the body increases in size, 
and the waste of its organs is repaired, is called nutri- 
tion. Its agents are supposed to be those minute ves- 
sels, that are situated between the termination of the 
arteries, and the commencement of the veins, and 
which are known by the name of capillaries. These 
vessels are distributed largely to all parts of the body, 
and have the power of separating from the blood parti- 
cles identical in character with those of which the 
various organs of the system are composed. 

It has been before remarked, that a species of com- 
position and decomposition is constantly going on in 
the body during life. The first of these is effected by 
the blood-vessels, and the latter by the absorbents. By 
digestion, a nutritive fluid, called chyle, is extracted 
from the food taken into the stomach; the lacteals con- 
vey this into the blood, and partially assimilate it to that 
fluid; but it is not yet fit for the purposes of life. It 
is carried by a distinct set of vessels into the lungs, 
where it parts with some of the noxious principles it de- 
rived from digestion, and it also receives others from 
the air, which, as it were, impart to it vivifying prop- 



124 NUTRITION, 

ertles. In this state it is returned to the heart, and 
this organ sends it through numberless vessels to every 
part, for their growth and nourishment. But the mere 
circulation of this fluid would not be sufficient; a por- 
tion of it must be left in each of the organs to supply 
the w^aste, and this is probably done by the minute ca- 
pillary vessels of the part. Under ordinary circum- 
stances, these vessels cannot be seen by the eye, even 
when aided by the microscope: they are so minute as 
to elude all examination in their natural state. But, 
small as they are, they are agents whose functions can- 
not be dispensed with in the animal economy. One 
set performs an important office in the lungs, as has 
been already noticed, and the other, spread throughout 
the system, is carrying nourishment to all the organs. 
The latter have received the name of the nutritive 
arteries. The nutritive process is a sort of secretion 
by which diffisrent substances are separated from a 
common fluid, the blood. Thus, one set of these vessels 
deposits the fibrin to form the muscles, and another, 
the earthy and animal parts of the bone. We are 
wholly ignorant how this is accomplished; but of the 
fact there is no doubt. 

Nearly all the parts of the body are continually, 
during life, subjected to this process; the old particles 
are taken up by the absorbent vessels, and new ones 
are deposited in their place by the nutritive arteries. 
The hair, the nails, the outer covering of the teeth, the 
coloring matter of the skin, and perhaps the cuticle, form, 
almost, if not the only exceptions. There is a great 
disproportion between the processes of composition and 



NUTRITION. 125 

decomposition, at the difFerent periods of life. Before 
the body has attained its full size, the function of nutri- 
tion is in great activity; a large amount of food is taken 
when compared with the size of the body; digestion is 
rapidly and easily performed, and the various organs 
are supplied with new particles, not only to supply the 
place of those that have been removed by the absorb- 
ents, but also to contribute to their growth. In the 
adult, the balance is restored between the nutritive ar- 
teries and the absorbents, the former, furnishing to the 
difFerent parts of the system, as much nutritive sub- 
stance as the latter remove, and no more. In old age, 
however, the absorbents have greater activity than the 
agents of nutrition; the body usually becomes smaller 
in health, the various parts are weaker, capable of less 
exertion, and offering less resistance to injuries. The 
bones become more brittle, from the absorption of some 
of their constituent principles, and break more easily at 
that period of life than at mature age. 

Every part of the body, as has been before remarked, 
is formed from the blood. The bones, though appar- 
ently so unlike this fluid, are secreted from it. If the 
food of a young animal be colored with madder, the 
bones will have a tinge of pink; if the animal take this 
food for a short time, then omits it, and then resumes it 
again, and so on, many times alternating in this way, 
it will be found, on killing the animal, and examining 
his bones, by sawing through one of the round ones, for 
example, that there will be concentric circles of white 
and pink osseous matter, corresponding with the manner 
in which the animal had been fed. This experiment 
11*= 



126 NUTRITION. 

furnishes a decisive proof that the bones are constantly 
undergoing this double process of composition and de- 
composition; and so universal is this throughout the 
system, that the body can never be said to be identically 
the same that it was at any given time, however short, 
before. This change takes place more rapidly in some 
organs than in others, and more slowly in old than in 
young subjects. Among those whose size and physical 
properties are altered with rapidity, may be named the 
skin, the muscles, and the glands; while the tendons, 
the cartilages, and the bones, are changed much more 
slowly. 

Many calculations have been made, to determine in 
what length of time the whole body is renovated; but 
no satisfactory results have been obtained. Some have 
supposed that it is accomplished in three years, and 
others have fixed the period at seven. This very dif- 
ference is enough to satisfy any one, that the calcula- 
tions are made without any data that can be depended on. 

We know that this renovating power exists in much 
greater activity in the young than in the old. The 
length of time necessary to repair the various injuries 
which the body receives, is very different in the two. 
The bones in childhood unite in a few days, while in 
old age they require weeks to accomplish it, and, in 
some instances, so feeble are the powers of nutrition, 
that it is never effected. 

It sometimes happens that the nutritive arteries have 
their action so much increased in some parts, as to pro- 
duce preternatural growth. Of this character, are some 
of those tumors, called wens, which not unfrequently con- 



NUTRITION. 127 

sist of mere fat, the ordinary adipose substance. They 
apparently originate, merely from an increased action of 
those vessels, whose office it is to deposit the fat; there 
are other tumors, however, which consist of substan- 
ces unlike any known to exist in the body. It some- 
times happens also, that the nutritive arteries of a part 
take on a new action, and deposit not their ordinary 
substance only, but others which they have not hereto- 
fore secreted, but which are formed by the vessels of 
other organs of the body. It is in this way only that 
we can account for the depositions of bony matter that are 
frequently seen, sometimes on the inner coat of the ar- 
teries, about the heart, and even in the brain. In some 
diseases, as gout and rheumatism, chalky deposits are 
very frequently made about the small joints. These 
are sometimes as large as pigeons' eggs, and they are 
oftentimes so superficial, that by merely removing a 
very thin layer of skin that covers them, they can be 
used, w^hen situated on the hand, for the purposes of 
marking or wi'iting, as well as a piece of common chalk. 
A question has arisen whether all the substances de- 
posited by the nutritive arteries previously existed in 
the blood. The experiments that have been made to 
settle this point, are somewhat contradictory. Dr. For- 
dyce found that if canary birds were not supplied with 
lime in some form, about the period of their laying, 
they usually died. This he attributed to their not 
being furnished with the materials necessary to form the 
shell of the egg, and that the egg could not therefore be 
perfected. To a certain number of canaries he gave 
lime, at the period of their laying, which they swal- 



128 NUTRITION. 

lowed greedily, and all did well; to others at the same 
period he gave none, and many of them died. From 
this it might be inferred, that the nutritive vessels have 
not the power of furnishing the lime for the shell, unless 
it has been previously taken into the system. But the 
experiment of Vauquelin led to a different conclusion. 
He found that a hen, who took, in a given period, but 
336 grains of calcareous and siliceous matter, formed 
during the same time 971 grains making a difference of 
615 grains that must have been formed by the nutritive 
vessels. 

It is very certain that no trace can be found in the 
blood of several of the secreted substances; but in those 
instances, in which these substances are of a compound 
character, it is not difficult to suppose that they are 
formed in the appropriate organs by a combination of 
their constituent elements. But this does not explain 
the formation by the secretory or nutritive vessels of 
some of the simple substances, or those which have 
usually been considered such, as lime and silica. Mod- 
ern chemistry, it is true, has succeeded in decomposing 
them; but this does not remove the difficulty, unless it 
should appear that their elements have been taken into 
the system, either in the food or some other way. Of 
this, there was no evidence in the experiment of Vau- 
quelin. It is impossible to say what new light further 
researches in animal chemistry may throw on this sub- 
ject; at present, however, it is involved in great ob- 
scurity. 

It was before stated, that the coloring matter of the 
skin was not subject to this process of composition and 



NUTRITION. 129 

decomposition; in other words, that it was not alter- 
nately deposited by the nutritive vessels, and removed 
by the absorbents, like most of the other parts of the 
body. This will perhaps explain the fact, which is fa- 
miliar to all, that the marks made on the skin by Indian 
ink and some other substances, are permanent. They 
are probably introduced into the texture, in which the 
coloring matter of the skin is deposited, and as this tex- 
ture has no absorbent vessels, there can be no way of 
removing the mark, but by cutting out the part. 

We see, too, on the skin of negroes, that the scars of 
wounds of any considerable depth, are uniformly of a 
white color. The texture, in which the black pigment 
is deposited, having been destroyed by the wound, there 
are no means of regenerating it, and the part is supplied 
with the ordinary skin. All that is known on the sub- 
ject of nutrition, may be stated in a few words. The 
various parts of the body, with but few exceptions, are 
furnished with nutritive arteries and absorbent vessels. 
The former have the power of depositing in the part to 
which they are sent, such substances as are adapted to 
their nutrition and growth. In cases of accident, by 
which any organ is injured, the nutritive vessels of the 
part take on an increased action, and deposit a much 
greater quantity of their ordinary secretion, in order to 
repair the injury. But how this is accomplished, or in 
what way vessels of apparently similar character, and 
circulating the same fluid, can deposit substances of 
very different characters in different parts, is wholly un- 
known. 



130 NUTRITION. 

Hitherto our attention has been engaged in the ex- 
amination of the functions of organic life, those functions 
which are connected with the organization and growth 
of the body, and which the animal has, to a certain ex- 
tent, in common with the vegetable. We have seea 
that the food is converted by the stomach into a homo- 
genous mass, from which the first intestine extracts a 
fluid, that becomes assimilated to the blood, and ulti- 
mately forms a part of it. This fluid, having been pre- 
pared by the digestive process, is taken up by means of 
another, which is called absorption, and poured into the 
blood-vessels. 

By means of the circulating system, it is first con- 
veyed to the lungs, for the purpose of undergoing a 
change that is effected in it by respiration, and then, by 
another part of this system, it is distributed to all the 
organs of the body. That function, by which the nU'- 
tritive parts are separated from the blood, and which is 
called secretion, has also been spoken of, and the 
various modes in which this function are performed 
have been noticed, and the purposes of the difl'erent 
secretions pointed out. 

It is apparent that all these processes are intimately 
connected with, and have a mutual dependence on each 
other; audit is also evident, that they have a common 
object, which is the nourishment and growth of the body. 

It seems wonderful that physiologists should seek to 
explain operations, so wholly unlike every thing that 
takes place in inert matter, by the known laws of chem- 
istry and mechanics; but such is the fact. Digestion 
was for a time supposed to be the effect of mechanical 



NUTRITION. 131 

pressure, and when the present theory was adopted, it 
was called the theory of chemical solution, and Spallan- 
zani, who did much to establish it, thought that it could 
go on, under certain circumstances, nearly as well out of 
the body as in. But facts do not bear him out; and 
though we may admit the power w^hich the gastric juice 
has of dissolving food, it cannot be denied that it is a 
power which is in a great measure dependent on vitality. 

Similar attempts have been made to explain the mode 
in which the other functions are carried on. The 
venous circulation has been said to be the result of the 
operation of the common principles of hydraulics; se- 
cretion has been called a sort of filtering; and the ab- 
sorbents, we are told, take up their fluids by capillary 
attraction. That wonderful and mysterious change, 
w^hich respiration produces in the blood, has been 
thought to be a mere chemical effect, and animal heat 
has been supposed to be the result of that process, by 
which this fluid is made to part with its latent heat. It 
is time that a different view was taken of this subject. 
There are other laws which have an influence in the 
animal economy, besides those of chemistry and me- 
chanics, and these are the laws of vitality. They are 
not yet as well understood as those of the physical 
sciences; but we see operations going on in the living 
animal body, that cannot be explained by any known 
physical laws, and which sometimes seem to be at va- 
riance with them. 

The functions of which we are next to speak, are 
those which connect man with external objects; and 
there is no ground for pretending that they have any 



132 NUTRITION. 

thing in common with those operations which are the 
effect of the laws of chemistry or mechanics. They 
are the functions which raise the animal above the 
vegetable, and which on the whole are enjoyed by man 
in a much higher degree than by any other created being : 
these are the functions of animal life. 

We shall first speak of the nervous system and its 
functions, then of the senses and their organs, next of the 
voice and its organ, and finally of locomotion and its 
agents. 



(133) 



CHAPTER VIII. 

OF THE NERVOUS SYSTEM. 

The nervous system includes the brain, the spinal 
cord or marrow, the nerves, and the ganglions. The 
brain is a large organ, of a soft texture, occupying the 
whole cavity of the skull. It is of an irregular rounded 
form, with numerous convolutions and prominences, 
some of which have corresponding depressions in the 
bone, as if the shape of the brain was made to conform 
to the skull, though in fact there is no doubt that the 
brain gives the form to the bone. On its outer surface, 
it is of an ash color: this has hence been called the 
cineritious portion, and, from its position, it has been 
known under the name of the cortical part, as if it 
formed a covering to the whole, like the bark of a tree. 
The inner part is white, like marrow, and is called the 
medullary portion. 

There are two great divisions of the brain; the larger, 
which is also the upper and front part, is called the ce- 
rebrum ; the other, which occupies the back and lower 
part of the skull, is called cerebellum. Both of these 
are divided to a considerable extent into two parts, on 
the right and left, called hemispheres. The hemispheres 
12 



134 



NERVOUS SYSTEM. 



are separated by a strong, membranous partition or 
septum. 

The brain has three membranes. One, which closely 
adheres to it, is of a texture so delicate, that it cannot 
be separated from it. The middle one has but litde 




The upper surface of the Brain. 



firmness; but in some parts, it is so loosely attached, 
that it can be examined much better than the other. 
The outer one, which is next to the skull, is a strong, 
elastic membrane, and affords great protection to the 



NERVOUS SYSTEM. 136 

brain. There are four cavities within the substance of 
the brain, the precise use of which is not known. 
Some have supposed that they were formed for the 
purpose of allowing more space for the blood-vessels. 
During life, a serous exhalation is constantly going on 
in their interior, and in health, this fluid is probably ab- 
sorbed nearly as fast as it is secreted. But it some- 
times happens, in a diseased state of the organ, that 
this exhaled fluid accumulates and produces that formi- 
dable disease, known by the name of internal dropsy of 
the brain, 

The cerebellum or smaller brain, is covered by a 
very firm membrane, which separates it from the cere- 
brum. The care with which the cerebellum is pro- 
tected from injury is sufficient proof, if there were no 
other, of its importance in the economy. It seems to 
be an object to guard it from violent jars, and in some 
animals who are compelled to leap for their prey, this 
covering, which is a membrane in the human species, in 
them is formed of bone. The skull, which is opposite 
to the cerebellum, is also much thicker than at any 
other part, and a fracture at this place is more likely to 
prove fatal, than any where else on the head. 

The brain is largely supplied with blood. Though 
in weight it constitutes but a fortieth part of the body, 
it receives, according to the calculations of some physi- 
ologists, a fifth part of the blood. This is probably 
exaggerated; but it is admitted by those who have 
doubted the correctness of this estimate, that at least a 
tenth part of all the blood is sent to it. It is certain that 
it is supplied by four large arteries; and the manner in 



136 NERVOUS SYSTEM. 

which these enter the skull, shows the necessity of de- 
fending the brain from any violent jar. The canals of 
these arteries are so curved, that the blood enters with- 
out producing any concussion of the organ. These 
great arteries freely communicate with each other soon 
afterentering the skull; an arrangement, which is made, 
no doubt, to prevent the brain from losing any part of 
its supply of blood by an accidental obstruction of one 
or more of its arteries, which might sometimes occur 
from injuries of the brain, or the growth of tumors in 
that organ. 

If the medullary part of the brain be hardened by ar- 
tificial means, and then carefully scraped, there will be 
an appearance of fibres, which seem to diverge from 
the base of the brain. It is not yet settled whether 
these fibres actually cross each other, so that those from 
one side pass over to the other. That this does take 
place, seems probable from a circumstance which re- 
cently came under my notice. A young man fell with 
a piece of wire in his hand. This wire was very 
pointed at one end, and entered his brain directly above 
his eye. Neither the eye nor the optic nerve was in- 
jured: his senses were unimpaired, but the motion of 
the opposite side of the body was destroyed, though the 
sensibility remained. Leeches and other applications 
were made to the head, and, after a few weeks, the 
motion of the leg was restored ; but that of the arm, 
though partially recovered, is still impaired, and it is 
now several months since the accident. This case cer- 
tainly favors the opinion that the nerves of one side of 
the brain supply the opposite side of the body, and 



NtRVOUS SYSTEM. 137 

render it probable, that the medullary fibres do actually- 
cross each other. 

At the base of the brain, there is a small projection, 
which is called the pineal gland. It is worthy of notice, 
because Descartes supposed it to be the seat of the 
soul. It having been found to contain a few grains of 
earthy matter in the brains of some idiots, who were 
examined after death, it was at once inferred, that this 
earthy substance was the effect of disease, and pro- 
duced the loss of mind. But it has since been ascer- 
tained, that it exists in the brains of all persons, though 
no one has as yet made a probable conjecture as to the 
use of it. By analysis it appears that this earth is 
principally lime. 

The spinal cord or marrow is contained in a bony 
canal formed by the bones of the spine. It is, as it 
were, a continuation of the brain, and resembles it very 
closely in its structure. It is covered by a firm mem- 
brane, and the canal which encloses the whole, is so 
strong, and so well supported, that the injuries of the 
spinal cord are not very frequent. 

The nerves are white cords of medullaiy matter, that 
are connected with the brain and spinal marrow. There 
are no less than twelve pairs from the brain, and thirty 
pairs from the spinal marrow. It has been usual, till 
recently, to say that there were but nine pairs of cere- 
bral nerves, or nerves of the brain, but the subdivision 
that has been made of some of them, and which in- 
creases the number to twelve, is now generally adopted. 
All the nerves are enclosed in a sheath. Their fibrous 
12* 



138 



NERVOUS SYSTEM. 



Structure is more evident than that of the spinal cord, 
or even than that of the brain itself. 

The nerves soon subdivide and communicate freely 
with each other, forming a sort of net-work, known by 
the name of plexus. New nerves go off from the 




Base of the Brain. 



plexuses, which seem to be independent of the original 
ones which entered into their structure. The nerves 
of the brain are principally distributed to the organs of 
sense, and those of the spine to the muscular parts. 



NERVOUS SYSTEM. 139 

The ganglions are small bodies of a reddish color on 
the exterior, but white within, that are found along the 
course of some of the nerves. They resemble some- 
what the brain in their structure, being composed of two 
parts of different colors- It is generally admitted that 
the nerves that go from them are larger than those that 
entered them; as if they imparted to the nerve some 
additional power. 

This cursory view of the structure of the nervous 
system, may perhaps be sufficient to render more intel- 
ligible the account that w\\\ now be given of its functions. 

The brain is the material agent of the mind, and it is 
also the organ which perceives the impressions that are 
made on the various parts of the body. The intellec- 
tual faculties and operations are so unlike the ordinary 
effects of matter, that it is not easy to believe that they 
are the result of any material organization. There 
must therefore be an immaterial something, which we 
call mind, and the brain is the instrument by which it 
operates. Of the mode of this mysterious connexion 
of mind and matter we know nothing; but w^e see abun- 
dant evidence of their reciprocal action on each other. 
Mental emotions and affections, if violent and-long-con- 
tinued, are not unfrequently productive of disease of the 
body, and a disordered state of the body oftentimes 
impairs or deranges the operations of the mind. 

There is unequivocal evidence, then, that the brain is 
the organ of intellect. So strongly have many been 
impressed with this belief, that they have supposed that 
the mind w^as nothing more than the result of the or- 
ganization of the brain; yet to this doctrine, the objec- 



140 NERVOUS SYSTEM. 

tion stated above seems to be unanswerable. But al- 
most every day brings us additional proof that the brain 
is the organ of the mind. A blow on the head, if it 
produces a depression of a portion of the bone, and a 
consequent compression of the brain, is followed often- 
times by a total loss of the intellectual faculties. If the 
depressed bone be removed by an operation, the mind 
is restored to its wonted vigor. 

Numerous cases of this kind are on record; but one 
of the most striking, occurred in the Mediterranean, in 
the year 1799. The patient was found on board the 
ship to which he was attached in a state of insensibility, 
but it was never ascertained how the accident occurred. 
He was first removed to Gibraltar, and thence to Dept- 
ford,and in May, 1800, was admitted into St. Thomas' 
Hospital, London. From the time of the accident to 
his admission into the Hospital, there was a complete 
loss of his mental faculties and bodily power. But he 
took food and digested it; the blood circulated freely, 
and the pulse was natural. His respiration was unaf- 
fected. On examination, it was found that a portion 
of the skull was depressed. This was raised by Mr. 
Cline. In a few hours, he gave indications of returning 
reason, and on the following day he spoke. In a short 
time he recovered entirely, and he had no recollection 
of any thing that that occurred from the time of the ac- 
cident till the operation, a period of a year. 

It was evident in this case, that the brain was the 
only part of the body that was affected, and that this 
affection consisted in a compression of the organ. It is 
certainly wonderful that this state of things having con- 



NERVOUS SYSTEM, 141 

tinued so long, did not produce some disease of the 
brain, of so serious a character as to prevent the re- 
covery of the intellectual faculties. 

The case of a beggar, who exhibited himself in Paris, 
has been frequently told. By some accident he had 
lost a portion of the skull; the brain at that place was 
covered only by its membranes and the common integ- 
uments, and for a trifling sum he would allow any one 
to press on this exposed part. As soon as the pres- 
sure was made to any extent, he became wholly un- 
conscious, but his intellect was immediately restored 
when the pressure was taken off. 

A violent concussion of the brain, without com- 
pressing or lacerating any part of it, is oftentimes fol- 
lowed by total insensibility. Patients sometimes re- 
main in this state many hours, and even several days, 
and yet ultimately recover. 

Injuries of no other part of the body, however severe 
they may be, produce the same effect on the mind. 
The intellect will remain unimpaired, so long as the 
functions of the brain are perfectly performed. 

What part of the brain has the most intimate relation 
with the intellectual faculties is not known. It is cer- 
tain tliat some portions are of less importance than 
others. The outer or cortical part is frequently wounded 
in accidents, and large portions of it are removed, 
without affecting the functions of the organ. Absesses 
and tumors form in the brain, and so long as they do 
not compress it, that is, so long as the absorption of 
the brain takes place as rapidly as the tumor or abscess 
is formed, the mind is unimpaired. It is not unfrequent 



142 NERVOUS SYSTEM. 

to see ^arge pieces, not only of the outer, but of the 
medullary part of the anterior and middle portions of 
the brain removed by injuries, without destroying the 
life or impairing the intellect of the patient. 

Though it may be considered as beyond controversy, 
that the brain is the organ of the mind, and that the in- 
tellectual functions will be more or less perfect accord- 
ing to the greater or less degree of perfection of the 
organ, yet we know not what constitutes that perfec- 
tion. Some have supposed that the mind is in propor- 
tion to the size of the brain, when compared with the 
size of the body. This opinion was advanced by Aris- 
totle, and has found advocates ever since his time. But 
this rule, though it may seem to be true in some in- 
stances, has so many exceptions, as to render it of no 
value. 

It is true that the brain of man, whose intellect is 
immeasurably beyond that of all other animals, is very 
large in proportion to the size of his body, when com- 
pared with that of some animals with whom we are fa- 
miliar. The human brain is four times as large as that 
of the ox, and yet the body of the ox is six times as 
large as the human body. But it has been found, on 
the other hand, that there are some animals, some spe- 
cies of singing-birds, for example, in whom the brain 
bears a larger proportion to the body than it does in 
man ; while some other animals, remarkable for their 
sagacity, as the elephant and the horse, have brains 
much smaller in proportion to the size of their bodies, 
than other animals of far inferior intellect. 

Besides, the brains of infants, at a period when 
scarcely the slightest trace of intellect is discoverable. 



NERVOUS SYSTEM. 14$ 

bear a much greater proportion to the size of their bod- 
ies, than they do in after life, when the mind is fully 
developed. 

The brain is also the organ which perceives the im- 
press ons that are made on all parts of the bo h . If, 
for example, the eye be ever so perfect, and the nerve 
which connects it with the brain, performs its functions, 
there will be no vision, unless the brain also acts. 
This is seen daily in disease. In dropsy of the brain, 
in which the origin of the optic nerve is compressed, 
though the nerve remains perfect and the eye uninjured, 
vision is lost. The brain is the centre of the nervous 
system; and all those actions originate in it which are 
performed by organs that are under the control of the 
will. It will be seen, that there is a set of nerves over 
which the will has probably no control, and this will 
explain the manner in which the various functions of 
circulation, digestion and respiration are carried on, 
when the brain is compressed, and the intellect gone. 

The nerves are the agents of sensibiHty ; that is, 
they convey to the brain the impressions which are 
made upon them. No other organs perform this office; 
but all the nerves have not this function, and some of 
them are destined for other important purposes. The 
usual division of the nerves has been into those of the 
brain, and those of the spine. There are twelve pairs 
of the former, and thirty of the latter. The nerves of 
the brain are distributed mostly to the organs of sense, 
and those of the spine to the muscles of voluntary mo- 
tion. The first nerve of the brain goes to the organ of 
smell, and is called the olfactory nerve. The second 



144 NERVOUS SYSTEM. 

is sent to the eye, and is the optic nerve, the nerve of 
sight. The third, fourth and sixth pairs of nerves go 
also to the eye, but have nothing to do with the power 
of vision. They are distributed to the vaiious muscles 
that move that organ. The fifth pair, which has three 
distinct branches, sends some of them to the eye, oth- 
ers to the nose, the jaws and the tongue. The nerves 
of this pair, which are sent, one to each side of the 
tongue, are probably the nerves of taste, or gustatory 
nerves. The seventh pair is the facial nerve, and is 
distributed to the muscles of the face ; the eighth pair, 
which, with the facial, was formerly called but one pair, 
is the auditory nerve. The ninth pair, generally known 
by the name of eighth, and also by that of the par-va- 
gum, is now called pneumo-gastric, because it sends 
branches to the lungs and the stomach, and it sends 
others also to the larynx and the heart. The tenth pair 
is distributed to the tongue and the pharynx, the upper 
part of the gullet, and it is thence known by the name 
of glosso-pharyngeal. The eleventh pair, formerly 
called the ninth, is the lingual nerve, and is sent to the 
tongue. From its passing under this organ, it is also 
called hypo-glossal. The twelfth pair arises from the 
spine, but passes out through one of the holes of the 
skull, and is distributed to the muscles of the neck. It 
was formerly called the spinal accessory nerve of Wil- 
lis. This last nerve, if we regard its origin, should be 
classed with the spinal nerves; but as it comes out with 
the nerves of the brain, it is ranked with those of that 
organ. 

None of the nerves of the brain originate in the cere- 



NERVOUS SYSTEM. 145 

brum or cerebellum, but all arise from what is called 
the medulla oblongata. This is a portion of the brain, 
situated at the base, and between it and the spinal cord. 
The olfactory, the optic, and the auditory nerves, give 
off no branches till they arrive at the organs which they 
are destined to supply. 

Thirty nerves on each side go off from the spinal 
cord, and all of them, like those of the brain, pass 
through bony canals. They are sent to the muscles of 
voluntary motion. It is necessary that the same com- 
munication should be kept up by the nerves, between 
the spinal cord and the parts to which its nerves are 
distributed, which is known to exist with regard to the 
brain. If the spine be fractured, so that the spinal cord 
is compressed, all the parts below the fracture are par- 
alyzed. Sometimes the injury is in the lower part of 
the back, and the patient is only deprived of the use of 
his lower extremities ; and though his recovery is hope- 
less, his life is not destroyed, and probably not short- 
ened. In those cases, where the spinal cord is injur- 
ed high in the neck, death does not always immediately 
follow ; and w^hile life continues, the mental faculties 
are unimpaired. 

Sir Charles Bell has proposed an arrangement of the 
nerves according to their functions, which is equally re- 
markable for its truth and beauty. He divides the for- 
ty-two nerves that have been spoken of above, into two 
classes ; those included in the first, he calls original, or 
symmetrical nerves, and those of the second, the su- 
peradded, or irregular. The symmetrical nerves are 
the thirty pairs of spinal nerves, and the fifth pair of the 
13 



146 NERVOUS SrSTEM. 

brain ; the irregular, or superadded, are the remaining 
nerves of the brain. 

The symmetrical nerves have double roots, one aris- 
ing from the hind part of the spinal cord, or medulla 
oblongata, and the other from the fore part. The pos- 
terior root has uniformly a ganglion. Those filaments 
which arise from the posterior root, form the nerves of 
sensation; and those from the anterior, constitute the 
nerves of motion. These nerves are given off with 
great regularity to each side of the body, and they are 
found in all animals, from the zoophyte to man. 

The irregular, or superadded nerves, always arise 
from a single root ; they are irregular in their distribu- 
tion, and are superadded to the other system in greater 
or less number, in proportion to the degree of perfec- 
tion of the organization of the animal, being found most 
numerous in man. 

It has long been known that the medulla oblongata 
and the spinal cord are composed of three distinct sets 
of fibres or columns, the anterior, the posterior, and the 
middle. The nerves arising from the anterior, are the 
nerves of motion; those from the posterior are the 
nerv^es of sensation; and those from the middle are the 
nerves of respiration, or rather of respiratory motion. 
Those nerves of the brain which arise by a single root, 
are destined to one function only, and this function, 
whether it be that of sensibility or motion, is determin- 
ed by the part of the medulla oblongata from which they 
arise. 

No difference can be discovered in the structure of 
the several kinds of nerves, in any part of their course; 



NERVOUS SYSTEM. 147 

and the functions they are designed to perform, can on- 
ly be known by ascertaining the place of their origin. 
Mr. Bell established many of the most important parts 
of his theory by very satisfactory experiments on liv- 
ing animals, and circumstances which are daily noticed 
in disease, tend very strongly to confirm his views. In 
the various forms of palsy, it is not unusual to see some 
parts deprived of all power of motion, while they at 
the same time retain their sensibility as perfectly as 
usual. This is remarkably illustrated in that paralytic 
affection, which is often seen in the face ; the nerve af- 
fected is the facial nerve, a nerve of motion ; and all 
the muscles to w^hich it is distributed, are paralyzed, 
though their sensibility is unimpaired. 

It w^ould not be proper, in a work of this character, 
to follow Mr. Bell in the various explanations he has 
given, in illustration of his views of the nervous sys- 
tem. It is enough to say, that they fully estabhsh the 
theory w^hich he has advanced, and that the division 
which he has made of the nerves, according to their 
functions, has already been attended with practical bene- 
fits of no small value. A surgeon, for example, of the 
present day, would not propose, as has been done, to 
divide the facial nerve, for that painful disease, called 
tic douloureux; for he would know that this was a nerve 
of motion, and its division would be productive of no 
other effects than a paralysis of the muscles of the face 
to which it was distributed. The knowledge which 
may be derived of the functions of the nerves by care- 
fully studying his theory, will be found useful in ex- 
plaining various phenomena of the body, both in health 
and disease. 



148 NERVOUS SYSTEM. 

The nerves then may be divided into four classes : 1st., 
those of sensation, including those of the senses, and those 
of coimnon sensation: 2d, those of motion: 3d, those of 
respiration or respiratory motion ; and to these must be 
added a fourth class, being those which neither convey 
sensation nor motion, nor have any agency in producing 
the respiratory motions, but which serve to connect the 
various parts of the body into one whole, and are es- 
sential to its nutrition, growth, and existence. This 
last division is sometimes called the ganglionic system 
of nerves, as numerous ganglions are found in its course. 
It is also called the intercostal nerve, because branches 
of it pass off between the ribs; and as it forms a bond 
of union between the different organs of the system, it 
is frequently known by the name of the sympathetic 
nerve. 

The origin of this nerve is somewhat doubtful. It 
has been supposed that the sixth pair of the brain fur- 
nished a small filament, which served as a rudiment for 
this system ; while it has been asserted by others, 
that its first branch came from the spinal cord. It is 
seen in the upper part of the neck, within the sheath of 
the carotid artery: it soon passes out of that, and runs 
down along the spine, giving out numerous branches to 
all the organs of the chest and the abdomen, and com- 
municating freely by small filaments with all the nerves 
in its neighborhood. The ganglions are very numerous 
along its whole course; so much so, in fact, that the 
nerve has been considered by many as a mere medium 
of communication between these different bodies. 

The functions of the ganglions are not well under- 
stood. Some have supposed that they are intended to 



NERVOUS SYSTEM. 149 

limit the nervous influence; while others have thought, 
with apparently more probability, that it is -rather their 
office to increase it. This seems probable from the 
fact that the nerves that go out of them, are larger than 
those that enter. Many have regarded them as smaller 
brains, independent centres of nervous power, formed 
for the purpose of placing the organs to the perform- 
ance of whose functions they contribute beyond the 
control of the will. This opinion is perhaps received 
with more favor at the present day than any other, and 
there are several circumstances that render it plausible. 
It is very certain that the organs which they supply, 
perform their functions without any act of volition on 
our part; in fact, they go on almost, if not quite, as well 
while we are asleep as when we are awake, and no ef- 
fort of our will can stop, retard, or interrupt them. All 
the secretions and all the various processes of nutrition, 
are wholly involuntary. 

From this superficial view of the nervous system, the 
importance of its office in the economy must be appa- 
rent. It is the agent of the mind, and unless well or- 
ganized, the intellectual faculties are imperfectly de- 
veloped. It is the organ of sensation, and imparts to 
other organs the power of motion; for the bones and 
the muscles are but the instruments. If the nerve of a 
limb be divided, every part below the division is power- 
less. The eye and the ear may be ever so perfect, 
but the one cannot see nor the other hear without their 
peculiar nerve. 

It is, in fine, the possession of the nervous system, 
that places animals above vegetables. The lowest 
13* 



150 NERVOUS SYSTEM. 

being in the animal kingdom lias some nervous filaments 
which distinguish him from a plant, that is endowed with 
mere organic life. And as we ascend in the scale of 
beings, we see that the nervous system is more liberally 
dispensed as we advance, till we arrive at man, who 
possesses it in the greatest degree of perfection. It is 
true that some of the inferior animals may have the 
nerves of one or more of the senses somewhat more 
acute than those of man, but his nervous power, as a 
whole, is vastly superior to that of any other animal. 

The elevated rank which man holds in the scale of 
being may be truly said to be owing to the exquisite 
structure of his nervous system. It has been justly 
observed, that all the ' faculties which elevate and dignify 
him, his reasoning power, his moral sense, his capaci- 
ties of happiness, his high aspiring hopes, are felt, and 
enjoyed, and manifested, by means of his superior ner- 
vous system. Its injury weakens, its imperfection 
limits, its destruction, humanly speaking, ends them.' 



(161) 



CHAPTER IX. 



OF THE SENSE OF SMELL. 

Among the functions which connect us with external 
objects, none have a greater influence than those of 
the organs of the external senses. There are five in 
number, viz. those of smell, taste, touch, hearing, and 
sight. 

The organ of smell is much more simple than those 
of some of the other senses, as those of hearing and 
seeing. The power of perceiving odorous bodies re- 
sides in the mucous membrane, called the pituitary or 
Schneiderian membrane, from the anatomist who des- 
cribed it, which lines the interior of the nostrils, and the 
cavities connected with it. There are four principal 
cavities; two situated in the upper jaw, and two in the 
prominent part of the frontal bone, directly above the 
eye. These cavities all communicate with the nostrils, 
and when odors of great pungency are applied to the 
nose, a sensation somewhat painful is felt, directly 
above the orbit of the eye. It sometimes happens, 
from inflammation and other causes, that the passage 
from the cavity in the jaw to the nostril becomes 
obstructed, and severe suffering follows. An opera- 



152 SENSE OF SMELL. 

tion becomes necessary: it consists in removing the 
tooth, which is situated directly under this cavity, mak- 
ing a perforation through the bone into it, and thus, 
relieving it of its contents. The teeth which are con- 
tiguous to this cavity, are the large teeth, or grinders; 
but on account of their situation, they are not formed 
precisely hke the corresponding teeth of the lower jaw. 
The lower teeth have but tw^o prongs or fangs, which 
are long, and approximate each other at their lower end. 
The teeth, however, in the upper jaw, which are oppo- 
site to this cavity, have three fangs, which are shorter, 
and which also diverge more than the corresponding 
teeth of the low^er jaw. The reason of this is apparent: 
if they were long, they would perforate the floor of the 
cavity of the jaw; and if there were only two short 
fangs, the tooth would not be sufficiently firm; but by 
having three, and all of them well separated, the teeth 
are as well secured as those of the lower jaw. This is 
an instance of that design which is so apparent in the 
construction of every part of the human frame. 

The organ of smell is largely supplied with nerves. 
This sense is supposed to reside in the first pair of 
nerves of the brain, called the olfactory, which passes 
out of that organ through a thin plate of bone, perforated 
with holes like a sieve, and through each of these holes 
a nervous filament passes. These filaments cannot be 
traced to every part of the membrane; they have only 
been found in the upper and middle portion of it. This 
has led some to suppose that the other nerves, the 
branches of the fifth, which are sent to the nostrils, 
were also olfactory nerves, but this seems to be 



SENSE OF SMELL. 153 

doubtful; for we know that the sense of smell is much 
stronger in the middle and upper part of the nostrils 
that it is lower down, if it resides there at all. It is 
probable that the other nerves are merely nerves of 
sensation, and nerves of communication; that is, they 
serve to connect this organ with other partsof the body. 
There is a connection between the nerves of the nose 
and the nerves of respiration: this is sometimes seen in 
sneezing, which is a violent and sudden action of the 
diaphragm, the great muscle of respiration. This action 
is not unfrequently produced by irritating the nostrils by 
almost any stimulus to which they are unaccustomed. 

The pituitary membrane is largely supplied with 
blood, and it is kept constantly moist by a mucus which 
is secreted on its surface. The tears also, which are 
not wanted for the purposes of the eye, are carried into 
the nostrils, through a passage called the nasal duct. 
This moist state of the membrane seems to be neces- 
sary to the perfection of the sense of smell: for it is 
always less acute, or disappears altogether if the mem- 
brane becomes dry from inflammation, or any other 
cause. This secretion is very copious, as evaporation 
is constantly going on with rapidity, from the air pass- 
ing through the nostrils in respiration. 

All odorous bodies throw off odorous particles. 
There is a great difference, however, among them in 
this respect; some throw them off under all circum- 
stances; others only when they are moist, or heated, or 
in motion, or rubbed. These particles unite with the 
air, and are carried in the act of inspiration through the 
nostrils, and in this way are brought in contact with the 



154 SENSE OF SMELL. 

membrane in which the sense of smell resides. The 
secreted fluid which moistens this membrane, is sup- 
posed to have not only the power of rendering it more 
sensitive, but also of entangling, as it were, the odorous 
particles, and thus detaining them longer in contact 
with the minute filaments of the olfactory nerve. 
Odors are not ordinarily perceived, unless we respire 
through the nostrils: there are some, however, so in- 
tense and pungent, as to affect the olfactory nerves, 
even if we do not inspire, unless we close the nostrils. 
But this is not usually the case: experiments show that 
if an opening be made in the wind-pipe, odors are not 
perceived so long as respiration is carried on through 
this artificial opening. 

There is some reason to believe that the sense of 
smell does not reside in the cavity above the eye, nor 
in that of the jaw, though they both open into the nos- 
trils and are Hned with apparendy the same membrane. 
In some cases, where these cavities have been open from 
accident or disease, experiments have been instituted 
which seem to have settled this point. Odorous va- 
pors were introduced into them, and unless they passed 
into the upper part of the nostrils, they were not per- 
ceived. There is, in fact, but little doubt that the sense 
of smell exists only in the upper part of the membrane, 
in that part to which the olfactory nerve is distributed. 
When an odor is faint, and scarcely, if at all, percep- 
tible, we draw the air forcibly through the nose, in the 
act of inspiration, and thus carry the odorous particles 
to the upper part of the organ. This we do instinc- 
tively; and the mere fact that instinct prompts us to do 



SENSE OP SMELL. 155 

it, is no small proof that it is the best way to accom- 
plish the purpose we have in view. The odorous par- 
ticles are in this way carried to the upper part of the 
membrane, which they might not otherwise reach, and 
strike it with some degree of force, and thus, as it 
were, make their presence felt. 

Notwithstanding these views, however, there are 
some distinguished physiologists who are of opinion, 
not only that the sense of smell exists in the cavities of 
the jaw, and the frontal bone, but that the fifth pair of 
nerves furnishes the olfactory, instead of the first, as is 
generally supposed. It is very certain that in those 
animals in whom the sense of smell is remarkably acute, 
as the dog and the elephant, these cavities are remark- 
ably developed. It is also certain, that in young chil- 
dren, in whom these cavities do not exist, the sense of 
smell is much less perfect than in after life. It is dif- 
ficult, however, to believe, after the experiments that 
have been made on the subject, that the sense of smell 
does reside in them, though there can be no doubt that 
they contribute, in some way or other, very materially 
to the perfection of this sense. 

Mr. Magendie has recently attempted to prove that 
the branches of the fifth pair of nerves are the olfactory 
nerves. But his experiments were made on living ani- 
mals, under such circumstances as would justify no 
conclusion upon the subject, and ought not to weigh in 
the least against the mass of evidence that exists in fa- 
vor of the old opinion. 

The use of the nose seems to be to conduct the 
odorous particles to the olfactory membrane. Sir 



156 SENSE OF SMELL. 

Charles Bell maintains, that the forn:i of this organ has 
nothing to do with the power of the sense. It is cer- 
tain, however, that if it be removed by accident or dis- 
ease, the individual is deprived of the sense of smell. 
Beclard states, what is certainly a very remarkable fact, 
that if he be supplied with an artificial nose, the sense 
will be restored. 

The smell in man is not a sense of a high order. It 
is a source of pleasure, enabling us to enjoy agreeable 
odors. But its loss is not sensibly felt, nor does it cut 
us off from many enjoyments. It is possessed in a 
much higher degree by many animals; and the well-au- 
thenticated facts that are on record on this subject, 
would appear almost incredible to those who have had 
no experience on the subject. It has been stated as a 
remarkable fact, tliat those animals who, like the birds 
of prey, for example, uniformly feed on the most pu- 
trid and offensive substances, have the sense of smell 
in the greatest perfection. But there can be no doubt, 
from the avidity with which these subsiances are seiz- 
ed, that though to us they may be ofiensive and dis- 
gusting, they afford to them the highest enjoyment. 
Throughout the animal kingdom, we find that every 
tribe is adapted to its situation, and by a merciful pro- 
vision, the moderate indulgence of the natural appetites 
is made a source of pleasure. 

The sense of smell is somewhat under the control of 
the will ; that is, we can use it more or less, according 
to the nature of the odors presented. We inspire for- 
cibly, to enjoy the perfume of a rose; and we breathe 



SENSE OF SMELL. 157 

cautiously, or close the nostrils, if an offensive odor is 
presented to them. 

This sense is, to a certain degree, susceptible of im- 
provement. Long practice will enable individuals to 
discriminate various substances by the odor, when, to 
others, they all emit the same. 

14 



(158) 



CHAPTER X. 

OF THE SENSE OF TASTE. 

The sense of taste resides in the mucous membrane 
of the tongue, the lips, the cheeks, and the fauces. 
Some have supposed that it exists in that of the gullet 
and stomach also ; but this is not correct ; for the 
nerve, which is now known to be the nerve of taste, is 
not distributed to these last-named organs. 

Though the tongue is an important organ of taste, 
it is by no means the only one, as it has sometimes 
been thought to be. In those persons who have had 
this organ removed, the sense of taste remains. It is 
probable, however, that in these cases, the other parts 
have an increased sensibility, in comparison with what 
they before possessed. The tongue is a double organ, 
composed chiefly of muscles. The two sides are so 
perfectly distinct, that it is not unfrequent, in palsy, for 
one of them to be paralyzed, while the other remains 
perfect. Its surface is covered with little prominences 
called papillae, which are supposed, by some, to have an 
erectile power ; that is, to be capable of raising them- 
selves, when savory food is brought in contact with 
them. The tongue is abundantly supplied with blood, 
having a large artery sent to each side of it. It is also 



SENSE OF TASTE. 159 

very largely furnished with nerves ; it receives branch- 
es from the fifth pair, besides the glosso-pharyngeal and 
the great hypo-glossal nerves, frequently called the 
ninth pair. Some uncertainty has existed, as to which 
of these was the gustatory nerve, or the nerve of taste. 
Recent experiments have proved satisfactorily, that the 
branch of the fifth pair, which is sent to the tongue, is 
the true gustatory nerve, and branches of it are distrib- 
uted to all those parts in the neighborhood in which the 
sense of taste resides. It is not merely the gustatory 
nerve, but it is also the nerve of common sensibility. 

The office of the glosso-pharyngeal nerve seems to 
be to establish a connexion between the tongue and the 
pharynx, the principal organ of deglutition, or swallow- 
ing. It is of obvious importance that these parts should 
act in concert ; and this can in no way be so well ac- 
complished, as by distributing the same nerve to the 
two organs. 

The hypo-glossal, or ninth pair of nerves, give to 
the tongue its power of motion. These nerves are 
very large, and the necessity of this is apparent, when 
it is recollected how numerous are the motions of the 
tongue, both in talking and in masticating our food. 

The membrane, in which the sense of taste resides, 
is constantly covered in health by a fluid that is secret- 
ed by small follicles, situated in the substance of it. 
This is distinct from the secretions of the salivary 
glands, which aid materially in mastication and degluti- 
tion of the food, though probably not in its digestion, 
while the mucus, that moistens the membrane of the 
tongue and mouth, is chiefly subservient to the sense 



160 SENSE OF TASTE. 

of taste. When this secretion is deficient, this sense 
is imperfect ; and it is lost, when the parts in which it 
resides are deprived of their usual moisture. 

This sense is closely connected with that of smell 
and that of touch. It resembles them both, in requiring 
to be in contact with the bodies, which produce the 
sensation of which it is the seat, and differs in this res- 
pect from the senses of seeing and hearing. It holds 
a much less elevated rank than the two latter. The 
enjoyments derived from it are strictly sensual and cor- 
poreal, and can in no way, like those of sight and hear- 
ing, be made to contribute to the enlargement of the 
mind. Its small importance, when compared with the 
two last named senses, may be inferred from the fact, 
that it is furnished with no nerve which is a nerve of 
taste only ; for the branch of the fifth pair, which is the 
gustatory nerve, is also the nerve of common sensibili- 
ty for the tongue. But the eye and the ear, on the 
contrary, in addition to an abundant supply of nerves of 
sensibilty, have also nerves, the optic and the auditory, 
which are exclusively nerves of sense. 

The sense of taste is usually very acute in children. 
They prefer food of the mildest character, and have an 
aversion to every thing that is strong or pungent. This 
sense, as is well known, is capable of being much mod- 
ified by habit : those articles which at first were dis- 
gusting, oftentimes become, by persevering in the use 
of them, highly agreeable. The use of tobacco is a 
striking example of this. 

In connexion with the sense of taste, it may be prop- 
er to say a few words of hunger and thirst, though 



SENSE OF TATSE. 161 

they might have been noticed when treating of diges- 
tion. 

Hunger is a peculiar sensation, seated in the stomach, 
arising from the want of food. When long continued, 
it is of a painful character. It has been explained, both 
on mechanical and chemical principles. It has been 
said to be owing to the opposite sides of the stomach 
rubbing against each other, when it was empty. A 
careful examination of the form and structure of this 
organ, would satisfy any one that this cannot take place; 
and even if it could, it is by no means probable, that 
the sensation produced by this friction, would at all re- 
semble that of hunger. 

The chemical theory is not much more plausible. It 
attributes the sensation of hunger to the corrosive ac- 
tion of the gastric juice on the inner coat of the stom- 
ach. But it is well known, that this fluid has no such 
action on any substance endowed with vitality. 

The only satisfactory explanation seems to be that 
which supposes it to be a specific sensation, produced 
upon the nerves of the stomach for a particular purpose. 

The sensation of thirst is usually thought to be seated 
in the mucous membrane, in which resides the sense of 
taste, and to be owing to a deficiency in the secretion 
of mucus which takes place in those parts. There is 
reason to beheve, however, that its seat is more ex- 
tended. An individual was wounded in the oesophagus, 
the passage which leads to the stomach; though his 
mouth was abundantly supplied with hquids, he suffered 

extremely from thirst. At length, fluid was conveyed 
14* 



162 SENSE OF TASTE, 

into the stomach through the artificial opening in the 
oesophagus, and the thirst ceased. 

Hunger and thirst are classed among those affections 
called appetites ; they are formed partly of a mental, 
and in part of a corporeal operation; they are designed 
to answer some important end in the economy, and 
they are brought about by the intervention of the ner- 
vous system. 



(163) 



CHAPTER XL 



OF THE SENSE OF TOUCH. 

This sense is more extended than any of the others. 
It resides in the skin and the mucous membranes. It 
has been proposed to distinguish the sense of touch from 
that of feeling ; but there does not seem to be sufficient 
reason to do this. They are both seated in the same 
organs, and have the same nerves, and the differences 
between them, appear to be only these: 1st, that the 
sense of feeling is more general, extending over the 
whole surface of the skin and mucous membranes, while 
that of touch is hmited to particular parts, being in man 
most perfect in the hand : 2d, that the sense of feeling 
is passive, while that of touch is active. 

As this sense resides principally in the skin, it is 
proper to give some account of tlje structure of this or- 
gan and its appendages. 

The skin is divided into three layers; the outer of 
which is called epidermis, or cuticle; the middle, the 
rete mucosum; and the inner, the true skin, or vera 
cutis. Some have added a fourth, which has been 
called the papillary body. But this has not as yet been 
sufficiently examined, to enable physiologists to decide, 



164 SENSE OF TOUCH. 

whether it should be considered a distinct layer, or only 
an appendage of the true skin, from which it arises. 

The epidermis is a semi-transparent membrane, 
which seems to be destitute of blood-vessels and nerves: 
it is certain that neither have been discovered in it, and 
that it is totally insensible. It does not appear to be 
organized, like the other parts of the body, and it resists 
putrefaction for a great length of time, hke inorganic 
substances. It contains numerous pores, but of so 
minute a size as not to be perceived by the strongest 
magnifying glasses. Yet there can be no doubt of their 
existence, and it must be through them that the per- 
spirable matter passes. 

The cuticle is seen to advantage, when a blister has 
been applied to the skin, and an effusion of water has 
taken place. The membrane that is raised up and con- 
tains the water, is the cuticle; and the fact that it can 
prevent the water from escaping, shows how very small 
its pores must be. 

The cuticle is supposed to be formed by the vessels 
of the skin: it is a kind of exudation or secretion, which 
is readily formed, as we see how soon it is repaired, 
when any part of it has been injured or destroyed. Its 
purpose is evidently the protection of the parts 
beneath it. It is thin and delicate in those places 
that are not much exposed to accident, while it becomes 
exceedingly thick and firm where there is much pres- 
sure, as in the soles of the feet. Sometimes from dis- 
ease, as scarlet fever and erysipelas, it comes off from 
the whole surface of the body, either in pieces of con- 
siderable size or small scales; and even in health, these 



SENSE OF TOUCH. 165 

scales are constantly separating, especially from some 
parts as the head, though more gradually than from dis- 
ease. It has been found to be composed chiefly of 
albumen. 

The rete mucosum or mucous net-work is situated 
directly under the epidermis. It was first described by 
Malpighi, who supposed that it gave the color to the 
skin. Its existence has been wholly denied by some 
anatomists, while others have asserted that it may be 
found in Africans, and all other nations of dark-colored 
skins, but not in the European; while others have main- 
tained the original opinion of Malpighi . The general opin- 
ion is now in favor of the existence of this layer, but its 
precise nature is not determined. Some have supposed 
it to be mere mucus, while others have thought, that it 
was a net-work of minute blood-vessels. Whatever it 
may be, it is certain that it is exceedingly delicate, so 
much so as not to prevent an intimate connexion between 
the cuticle and the skin. 

The true skin is a membrane of considerable thickness, 
elasticity and strength. It is copiously supplied with 
blood-vessels and nerves. It is endowed with great sensi- 
bility, and though it varies in this respect in different parts 
of the body, it is upon the whole greater than that pos- 
sessed by any other organ. In surgical operations, it 
is uniformly found that the patients suffer most pain 
when this part is cut through. When the skin is ex- 
amined with a magnifying glass, small prominent points 
may be seen on its surface, which are called papillae, 
and which some have proposed to consider as a distinct 
layer, under the name of papillary body. These papil- 



166 SENSE OF TOUCH. 

lae are thought to be the termination of nerves, and the 
seals of sensation. The skin is connected on its outer 
surface with the cuticle, and beneath with the cellular 
membrane, with which it is so intimately blended, that 
it is not easy to say, where one terminates and the 
other begins. In addition to its numerous nerves and 
blood-vessels, it is abundantly furnished with absorb- 
ents, and the processes of exhalation and absorption go 
on in the skin to a great extent. 

The skin is composed principally ofgelatine, and on this 
account it is used in the manufacture of glue. When gela- 
tine is united to tannin, a substance is formed which water 
cannot penetrate, and it is on this principle that leather 
is made. The skins are cleaned of their hair and other 
substances by lime-water, and then immersed in a strong 
solution of bark, which contains a large portion of tannin; 
a chemical union takes place between this and the gela- 
tine of the skin; and if the process be well-conducted, 
and the skin be allowed to remain for a long time im- 
mersed in this solution, a leather is formed which will 
effectually resist water. 

The nails and hair may be considered as appendages 
of the skin. The nails are intimately connected with 
the epidermis, and seem as if they were formed by an 
intimate union of several layers of this membrane. They 
are insensible, and their use is to support the pulpy 
part of the finger, when employed in the senses of touch. 
They are composed principally of albumen. 

Each hair arises from a bulb or root situated under the 
true skin, which it passes through, as it does also through 
the epidermis. It is composed of two parts, an outer 



SENSE OF TOUCH. 167 

covering, uhich is a tube, and a pulp which is con- 
tained in it. The hair, next to the bones, is the most 
indestructible part of the body. It possesses no blood- 
vessels, and it is destitute of sensibility. It has been 
thought by some, that in some forms of disease, blood- 
vessels enter it, and that it also becomes highly sensi- 
ble. But the more prevalent opinion now is, that the 
blood-vessels do not extend beyond the bulb, and that 
the pain which is experienced when this is diseased, 
arises from its increased weight acting on the parts to 
which it is attached, and which, no doubt, are in a state 
of irritation. 

It is not yet determined whether the color of the hair 
resides in the pith, or in the horny external tube that con- 
tains it. 

The sense of touch has been thought to be the most 
certain of all the senses, because the objects which 
come under its cognizance, must be in contact with its 
organs. By it we require a knowledge of the physical 
properties of bodies, their shape, dimensions, consist- 
ence, weight, &c., and by it alone we are able to judge 
of temperature. 

The sense of feeling resides in every part of the skin 
and many parts of the mucous membrane. The only 
thing necessary for its exercise, is that the body, on 
which it is to be exerted, should be in contact with it. 
No effort of the will is required ; the sensation is pro- 
duced as soon as the substance touches the organ of 
feeling. Though we obtain a knowledge of the tem- 
perature of bodies by the sense of feeling, we cannot 
acquire by it a correct idea of the actual heat of bodies. 



168 SENSE OF TOUCH. 

Thus, any thing will feel warm to us, if it be warmer 
than the atmosphere, though it be in fact colder than 
our bodies. Cellars seem warm to us in winter and 
cold in summer, though their temperature is nearly the 
same during the year. The sense of feeling, therefore, 
gives an idea of the relative temperature of bodies, but 
not of their actual. This sense is not equally acute in 
all parts of the body. It is most perfect where the 
epidermis is the thinnest, as tlie impression must be 
made through this covering on the nerves beneath it. 
The great object of this covering appears to be, to 
blunt the impressions which foreign bodies would make 
on the nerves of the skin; and in those parts, as in the 
soles of the feet, where it seems desirable that but little, 
if any impression, should be made, the epidermis is very 
thick and firm. 

The hand is the principal organ of touch, which re- 
sides in the greatest perfection in the extremities of the 
fingers. This organ is admirably adapted to its purpose. 
It is covered with a delicate epidermis, and abundantly 
supplied with blood-vessels and nerves. It has also 
numerous tendons, which enable it, under the guidance 
of the will, to execute an almost infinite variety ol mo- 
tions. When not in action, the hand has only the sense 
of feeling; but it is necessary to apply it to other bodies 
to bring into exercise the sense of touch. 

IMany, both among the ancients and the moderns, have 
been so much struck with the perfect mechanism of 
the hand, as to attribute to this source man's great su- 
periority over other animals. But this opinion is not 
well founded. The hand is as perfect in the idiot as in 



SENSE OF TOUCH. 169 

Other indlvidua^^;, and when it is destroyed, other organs, 
as the feet, acquire in a great degree most of its powers. 
The conclusion at which Galen arrived many centuries 
ago, seems to be the true one: that man had hands given 
to him, because he was the wisest creature; but he was 
not the wisest creature because he had hands. 

The nerves which supply the sense of touch are the 
common nerves of sensation. 
15 



(170) 



CHAPTER XII. 

OF THE SENSE OF HEARING. 

The organ of hearing is one of the most complicated 
in the human body. The precise office of the different 
parts is not yet known; it will be proper, however, to 
describe these parts in a general way, at least, in order 
to render intelligible what will be hereafter said on the 
sense of hearing. The different parts connected with 
this sense have been studied with much attention by 
anatomists; they have described them very minutely 
and distinguished them by names, not always very ap- 
propriate, but they have not been able to throw much 
light upon the subject of their use. 

The organ of hearing may be divided into the follow- 
ing parts, viz. the outer, the middle, and the inner part, 
and the auditory nerve. The outer part consists of the 
external ear, and the tube which leads to the membrane 
of the tympanum. The external ear is composed 
chiefly of cartilages, covered with a delicate skin, and 
largely supplied with nerves and blood-vessels. It has 
several prominences and depressions, each of which 
has a distinct name, and also some muscular fibres. It 
has been thought that these muscles, if they had not 



SENSE OF HEARING. 171 

been prevented from acting at an early age by bandages, 
&c. applied to the head, would have been under the con- 
trol of the will, and been able to move the external ear, 
and shut and open the entrance to the passage at pleas- 
ure. But this is not probable, for savages, whose 
heads are not subjected in infancy to any of the re- 
straints which civilization imposes, have not this power, 
nor do w^e observe it in infants before any application is 
made to them. 

The external ear, when well formed, inclines a little 
forward, and is admirably adapted to collect sound, 
which it transmits through the tube that leads to the 
membrane of the tympanum. This tube is nearly an 
inch in length, and is formed in part of cartilage, and in 
part of bone. It has a number of small glands or folli- 
cles which secrete the wax, and its entrance is guarded 
by stiff hairs, to prevent insects and other foreign bodies 
from entering. These, however, do not always answer 
the purpose for which they seem to be designed, for 
insects sometimes get into the ear and produce great 
pain. This pain is owing to the extreme sensibility of 
the membrane of the tympanum; but when it is recol- 
lected that this membrane has no opening, it must be 
apparent that the apprehension that is often expressed 
lest the insects should penetrate farther, is wholly 
groundless. 

The middle part of the organ of hearing embraces the 
tympanum and its membrane, the small bones of the ear, 
or ossicles, as they are called, and the Eustachian tube. 
The membrane of the tympanum is situated at the bot- 
tom of the external passage or tube, and is covered on 



172 SENSE OF HEARING. 

its exterior by a thin delicate skin, the same that lines 
the tube. Its inner surface is covered by a ntiucous 
membrane, and a nerve, known by the name of the 
chord of the tympanum, passes over it. To this inner 
surface also is attached one of the small bones of the 
ear. This membrane is placed obliquely, inclining 
downwards and inwards; it is tense, thin, and transparent. 
The tympanum is a cavity situated between the ex- 
ternal and the internal ear. It is of an irregular, cylin- 
drical form, with several openings, some communicating 
with the internal ear, and one which is the termination 
of the Eustachian tube. It also contains the four little 
bones of the ear, known by the names of the hammer, 
the anvil, the round bone, and the stirrup. These 
bones are all connected together: the end of die ham- 
mer is attached to the membrane of the tympanum, 
and the stirrup is placed over an opening which leads to 
the internal ear. Muscles of very small size are inserted 
into these bones, and move them in various directions. 

The Eustachian tube leads from the back part of the 
throat, or fauces, as they are called, to the cavity of the 
tympanum. It is about two inches in length; partly 
bony and partly cartilaginous. It is lined by a mucous 
membrane. Its two extremities are not of the same 
size, the one opening in the fauces being somewhat 
larger than the other. 

The internal ear is situated in a part of the temporal 
bone, near the base of the skull, which, from its stony 
hardness, has been called the petrous portion. It is 
composed of three parts, the cochlea, the vestibule, and 
the semicircular canals. The cochlea is so called from its 



SENSE OF HEARING. 



173 



resemblance to the shell of a snail. It is situated near 
the entrance of the Eustachian tube, and is the most an- 
terior part of the internal ear. It communicates with 
the cavity of the tympanum and the vestibule. The 
vestibule is situated in the central part of the internal 
ear, and is, as its name imports, a sort of porch or en- 
try, which communicates with all the other parts. By 




A Map of the Ear. A The external auditory tube. B The membrane 
of the tympanum. C The Eustachian tube. D The hammer. £ The 
anvil. F The round bone. Q The stirrup. H The oval opening. 
/ The semicircular canals. J The vestibule. K The cochlea. 

means of the oval opening, (the foramen ovale) it com- 
municates with the tympanum, and over this opening 
is placed the small bone called the stirrup (stapes) . It 
has communications also with the cochlea, the serai- 
15* 



174 SENSE OF HEARING. 

circular canals, and internal auditory tube, — the one 
through which the auditory nerve passes to the internal 
ear on its exit from the brain; and it is through the 
openings which lead from the vestibule to the internal 
auditory tube, that the branches of the auditory nerve 
go to the various parts of the internal ear. 

The three semicircular canals are situated behind 
the cochlea and the vestibule, and they all terminate in 
the latter. There is found in them a dark greyish semi- 
fluid substance, the use of which is unknown. 

The auditory nerve is a branch of the seventh pair, 
called the soft portion, or portio mollis: the other, which 
is distributed to the face and known by the name of the fa- 
cial nerve, is called the hard portion, or portio dura. The 
auditory nerve passes into the internal auditory tube, and 
is subdivided into numerous small filaments, which pass 
through minute openings, and finally terminate in the 
form of a pulp in the various parts of the internal ear. 

Hearing is that function by which we obtain a know- 
ledge of the vibratory motions of bodies. When these 
motions are produced, undulations in the air follow, 
which are propagated in all directions, and which, when 
they strike upon the ear, cause what we call sound. 
Air is the most common vehicle of sound, though all 
elastic bodies, whether solid or fluid, are capable of 
conveying it. Water is a better conductor of sound 
than air, and some solids better than either. If two 
stones, for example, be struck together under water, a 
person whose head is under the surface may hear the 
sound at a great distance. If a slight scratch be made 
on one end of a long stick of timber, and the ear be 
apphed to the other, it will be heard very distinctly. It is 



SENSE OF HEARING. 175 

a knowledge of this principle, of the propagation of 
sounds by solids, which has led to the formation of what 
are called stethoscopes, the instruments which are now 
used by physicians to obtain farther light on various 
diseases. One end of them is placed on the surface 
of the body over the diseased part, and the ear is ap- 
plied to the other; the sound that is propagated through 
them is carefully noticed, and an opinion is formed from 
the character of this of the probable nature of the affec- 
tion. It is obvious that these instruments are not 
adapted to all cases; they are chiefly used in affections of 
the organs of the chest, the heart, and lungs. 

The vibrations of a sonorous body will not affect the 
sense of hearing, — or, in other words, will not produce 
sound, unless they take place in a medium that is able 
to propagate them. Thus, if a bell be struck under the 
exhausted receiver of an air-pump, no sound follows. 

Though sound is enfeebled by passing from one 
medium to another, still it can be propagated from the 
air to the water. If a musket be discharged over a 
person, who is under water, he will hear the report, if 
he be within a moderate distance. But a question has 
arisen, whether sound when thus propagated from the 
air to the water, can be propagated from the water to 
the air again. As for example, whether the discharge 
of a musket over a diving bell covered with water could 
be heard by the persons in the bell; that is, whether the 
sound could be propagated from the air to the water, 
and from the water to the air in the diving-be 1? With 
a view of deciding this question, I descended in com- 
pany with three other persons in a diving-bell, twenty 
feet below the surface of the water, and remained there 



176 SENSE OF HEARING. 

more than forty minutes. While we were in the bell, 
a musket was several times discharged directly over it, 
but we heard not the slightest sound. A musket was 
again discharged, when we were ascending : at the mo- 
ment the top of the bell rose above the surface of the 
water, we heard the report distinctly, and even thought 
it nearly as loud as usual. 

Sound is capable of being reflected, and the angle of 
reflection has been ascertained. An echo is produced 
by a reflected sound, and it is on a knowledge of this 
principle of <he reflection of sound, that whispering gal- 
leries are constructed. The same remark applies also 
to ear trumpets. 

The velocity with which sound travels can be readily 
estimated. Repeated experiments have shown that it 
is, under ordinary circumstances, at the rate of eleven 
hundred and forty-two feet in a second. A knowledge 
of this fact enables us to decide with ease as to the dis- 
tance of a thunder-cloud during a thunder-shower. If 
the interval that elapses between the flash of the light- 
ning and the report of the thunder be carefully noted, 
the calculation is of course easily made. It is very 
common, during the discharge of artillery, to see the 
flash some seconds before the report of the cannon is 
heard. In this way, the distance of a ship of war at 
sea has been determined by those on board the vessel 
she was in pursuit of, and the expediency of coming to, 
or striking has been decided. 

The precise ofSce of all the different parts of the or- 
gan of hearing is not known. The purpose of that part 
of the external ear, which projects from the head., 



SENSE OF HEARING. 177 

seems to be to collect sound; and it is well adapted for 
this purpose, and for reflecting it into the external tube. 
It is not, however, essential to hearing, as it has been 
removed, in man and some other animals, without de- 
stroying or materially impairing this sense ; and some 
animals, as birds and moles, are destitute of it, and yet 
have the sense very perfectly. 

The external tube simply transmits the sound, w^hich 
has been received by the external cartilage. The 
wax, which is found on its surface, is supposed to be 
intended for the same purpose as the hairs placed at its 
entrance; that is, to prevent the iiuroduction of foreign 
bodies into it. 

Great pains have been taken to ascertain the use of 
the membrane of the tympanum. It would seem to be 
important, from its situation, and from its connexion 
with the other parts of the organ. It is tense and dry, 
and therefore w^ell calculated to transmit the impressions 
that are made upon it to the air contained within the 
tympanum, and thence to the various parts of the in- 
ternal ear. It possesses great sensibility, and has a 
nerve running across it, as has been before stated, call- 
ed the chord of the tympanum. Muscular fibres have 
been discovered upon it, and Sir Everard Home, who 
first described them, suggested that the membrane might 
be the part appropriated to the reception of musical 
sounds. But this hypothesis was soon overthrown, by 
a communicalion made to the Royal Society, by Sir 
Asdey Cooper, in which he stated, that a patient, who 
had the membrane of one ear entirely destroyed, and 
that of the other much injured, retained his power of 



178 SENSE OF HEARING. 

perceiving musical sounds unimpaired. It is certain, 
that whatever may be its functions, they continue to be 
perfectly performed after it has been ruptured. Many- 
persons can force smoke from the mouth, through the 
ear, which they could not do, if the membrane of the 
tympanum was whole, and at the same time they pos- 
sess the sense of hearing in full perfection. 

The use of the tympanum is supposed to be to trans- 
mit to the internal ear the vibrations made on the mem- 
brane. This is done in part by the air which it con- 
tains, in part by its walls or parietes, and above all, by 
the chain of small bones, that were spoken of before. 
One of these, the hammer, or malleus, is attached to 
the membrane of the tympanum at one of its extremities, 
and at the other, to the anvil or incus : this latter bone 
is connected with the round bone, or os orbiculare ; and 
this again with the stirrup, or stapes, which is placed 
over the oval opening, the foramen ovale, that leads to 
the vestibule. When, therefore, the membrane of the 
tympanum is acted upon by the vibrations of the air, it 
is thought that these bones are moved in such a way as 
to communicate these vibrations to the internal ear. 
But it is evident, that our knowledge on this subject is 
not very precise, from the fact that all these bones, ex- 
cept the stapes, may be lost, as they sometimes are 
from disease, and the individual will for some years af- 
ter retain the sense of hearing. 

The use of the Eustachian tube is more apparent. 
It is to admit the air to the cavity of the tympanum, 
and thus keep its membrane in a proper state of ten- 
sion, and render the pressure on both sides equal. It 



SENSE OF HEARING. 179 

undoubtedly performs an important part in the functioQ 
of hearing. If it be completely closed, deafness en- 
sues. This explains why we are deaf, from a cold in 
the head. The end of the Eustachian tube, which 
opens in the back part of the throat, is obstructed by an 
inflammation of its lining membrane, and the air conse- 
quently is not freely admitted into the cavity of the 
tympanum. Many cases of deafness arise from a per- 
manent obstruction of this tube. A knowledge of this 
fact, led Sir Astley Cooper to propose an operation 
for this species of deafness. He knew that many per^ 
sons heard perfectly well, though the membrane of the 
tympanum had been ruptured ; and he believed that 
the deafness from an obstructini of the Eustachian 
tube, arose from the circumstnnce that the air was not 
admitted to both sides of the membrane of the tympa- 
num. He therefore proposed to puncture this mem- 
brane, and he performed the operation several times. 
At first, it was thought to be attended with some de- 
gree of success ; but the deafness soon returned, in 
nearly if not all the cases, and the operation is now 
abandoned. 

But little is known of the use of the internal ear. 
Books on physiology abound with specu ations on this 
subject : distinct offices are assigned to the different 
parts, but it is the result of conjecture merely. Many 
have supposed that the cochlea performed a very impor- 
tant part in the function of hearing ; but Vicq-d'Azyr, in 
a memoir on the ears of birds, maintains that they are not 
furnished with it, and yet many of these animals possess 
this sense in a high de.ree. All that is certainly known 



180 SENSE OF HEARING. 

respecting the internal ear, is, that the auditory nerve in 
a soft, pulpy state, is expanded in the semicircular ca- 
nals, the cochlea and the vestibule; and it is probable, 
that, from their bony structure, they have the power of 
modifying the vibrations made on the membrane of the 
tympanum, which might otherwise be too powerful. 

The auditory nerve, which is a branch of the seventh 
pair, is a nerve of special sensibility, destined to this 
one purpose only. It has been ascertained by experi- 
ments, that it is entirely destitute of ordinary sensibili- 
ty; that it can be cut, pricked or torn, without produc- 
ing any pain. The nerves which furnish the ear with 
the ordinary sensibility, are derived from the fifth pair. 

The sense of hearing is next in importance to that of 
sight. Considering the ear merely as the instrument 
which makes us acquainted with sound, its place can- 
not be supplied by any other organ. It opens to us 
sources of intellectual enjoyment, thus furnishing an- 
other example of the goodness that directed the forma- 
tion of our bodies, by making the ear tributary to our 
pleasure, while it is at the same time instrumental to 
our comfort and convenience. This sense becomes 
much more acute, when that of sight is impaired or de- 
stroyed. By it, the blind judge with great accuracy of 
the distance of bodies in motion, the width of streets, 
and the height of buildings. It is capable of improve- 
ment, even when all the other senses are perfect. By 
habit, we learn to judge of the distance and direction of 
bodies, by the sense of hearing ; and the savage, who 
has improved this sense to a high degree, in pursuit of 
his enemy or his game, will distinguish sounds that are 
inaudible to civilized man. 



(isi) 



CHAPTER XIII 



OF THE SENSE OF SIGHT. 

The eye is the organ of sight ; but in order to give 
a correct idea of the function of vision, it will be neces- 
sary not only to describe this organ, but its various 
appendages. The apparatus of vision is somewhat 
complicated; but the uses of the various parts are much 
better understood than those of the organ of hearing. 

The eye is an optical instrument of the most perfect 
construction. It is of a globular form, composed of a 
number of humors, so called, which are covered by 
membranes, and enclosed in several coats. These hu- 
mors are called the vitreous, the crystalline, and the 
aqueous. The vitreous, which takes its name from its 
resemblance to melted glass, is situated in the back part 
of the eye, and constitutes the greater portion of the 
globe. It is of the consistence of the white of an egg, 
and is contained in numerous small cells, formed in a 
membrane of great delicacy, which also covers it. On 
its anterior surface, there is a slight depression, and in 
this is situated the crystalline humor or lens. This is 
a body of considerable thickness and strength ; it is in 
the form of a double convex lens, the convexity of the 
16 



182 SENSE OF SIGHT. 

two sides, however, is not the same. It is placed in a 
perpendicular direction, immediately behind the pupil, 
and is kept in its situation by a membrane, which is 
called its capsule. When macerated, it has been found 
to consist of several layers, which increase in thickness 
towards the centre. The analysis of Berzelius has as- 
certained that the materials of which it is composed, 
contain the same properties as the red globules of the 
blood, without the coloring matter. 

In front of the crystalline lens, and occupying the 
whole of the anterior part of the eye, is the aqueous 
humor, the only one of the three which is entitled to 
the name of humor. It is composed principally of 
water, with a few saline particles, and a very small por- 
tion of albumen. 

A curtain, with an opening in its centre, floats 
in the aqueous humor, but is attached to one of the 
coats of the eye at its circumference. This curtain is 
called the iris, and the opening in it is the pupil. It 
derives its name from the various colors it has in dif- 
ferent individuals, and it is the color of the iris that de- 
termines the color of the eye. There is considerable 
diversity of opinion as to its intimate structure : some 
have thought it to be a mere continuation of one of the 
coats of the eye, others have supposed it to be a pecu- 
Har texture, and others again are of opinion, that it is 
formed in part from one of the coverings of the eye, 
and that it has also a layer peculiar to itself. The 
back part of the iris is called the uvea. The iris di- 
vides the space between the crystalline lens and the 
front of the eye into two parts, called the anterior and 



SENSE OF SIGHT. 183 

posterior chambers, the former of which is much larger 
than the latter. All the light admitted to the eye, 
passes through the opening in the iris, and it is supplied 
widi muscular fibres, by which this opening can be di- 
lated and contracted. 

The eye has three coats or coverings. The outer, 
which is called the sclerotic, is a firm, fibrous mem- 
brane, w4iich serves to defend the eye from injury, and 
into which the muscles, that move it in various direc- 
tions, are inserted. It extends over the whole of the 
eye, except the fore part, where a transparent mem- 
brane, called the cornea, is situated. It is the scle- 
rotic coat that is known by the name of the white o 
the eye. 




A Sectiok of the HomaK Eye. a The aqueous humor, h The crystal- 
line l-^ns. c The vitreous humor, d is an object from which the rays ot 
light go off, and as tliey enter the eye, they are refracted by the diflferent 
humors, and form an inverted image, e, on the retina. 

Within the sclerotic coat is situated the choroid coat. 
It is a thin, delicate membrane, composed in great 
measure of blood-vessels and nerves. It is loosely at- 
tached to the sclerotic coat, which it covers, and is of 
the same form and extent. On the surface of the cho- 
roid coat is found a dark substance, called the black 



184 SENSE OF SIGHT. 

pigment, which is of great importance in the function 
of vision. 

The inner coat of the eye, if it be not an expansion 
of the optic nerve, is composed of nervous filaments, 
and is called the retina. It is of the same extent as the 
other coats, surrounding the whole globe of the eye, 
except the circular opening in front. To the edge of 
this opening, the circumference of the iris is attached; 
— the band which connects it is known by the name of 
the ciliary ligament — and over it is placed a convex, 
transparent membrane, called, from its resemblance to 
horn, cornea. It was formerly supposed that this was 
but an elongation of the sclerotic coat : it has been, 
however, ascertained, that this is not the case, and that 
it can be separated from it by maceration. 

The optic nerves are the second pair of the brain. 
Before they enter the eye, they come in contact, their 
fibres seem to intermix ; and it has been thought, by 
some, that the nerve of one side is sent to the eye of 
the opposite side. This, however, is not the case. 

The optic nerve does not enter the eye in the cen- 
tre, but at a short distance from it on the side towards 
the nose. It is not the nerve of sensibility; it is des- 
tined to one function only, that of sight. The eye is 
abundantly supplied with nerves, both of sensibility and 
motion, from other sources. 

The importance of the eye is obvious, from the 
manner in which it is guarded from injury. It is situ- 
ated in a deep, bony socket, with a prominence above, 
on which is placed the eyebrows : it is furnished with 
lids, which can be shut so closely, as to exclude not 



SENSE OF SIGHT. 185 

only foreign bodies, but even the light, when it is too 
intense. There is also an apparatus for the formation 
of tears, with which the eye is washed, and by which 
foreign particles are removed from it. 

The eyelids are two in number in man. They have 
a thin, delicate skin on the outside, muscular fibres be- 
neath, and a cartilage on their edges. They are Hned 
by a mucous membrane, which passes from them over 
the anterior part of the eye, which is called the tunica 
conjunctiva, because it is the tunic which connects the 
eyeball with the lids. It is loosely attached to the lids, 
so as to allow free motion in all directions. 

In the edge of the lids are numerous small glands, 
which secrete an unctuous substance, that is probably 
expended on the eyelashes. 

The tears are secreted by a small gland, called the 
lachrymal gland, situated within the orbit of the eye, at 
its outer angle. This secretion goes on while we are 
asleep, as well as when we are awake. The eye is 
kept constantly moist. When the tears have perform- 
ed their office, they pass through two small openings, 
one in each eyelid, called puncta, and are thence con- 
veyed into the nose. It has been usual to call the up- 
per part of this canal, the nasal sac, and its continua- 
tion, the nasal duct. But there seems to be no reason 
for this distinction, as the canal is of nearly the same 
size throughout its whole extent. It not unfrequently 
happens, that this canal, from inflammation and other 
causes, becomes obstructed, particularly in that part 
that leads into the nose. The tears flow over the 
cheek ; the upper part of the canal swells, inflames, 
16* 



186 SENSE OF SIGHT. 

and finally bursts, and in diis state it constitutes the 
disease which is known by the name of fistula lachry- 
nialis. 




a The lachrymal gland, b Its several ducts, to con- 
vey the tears to the eye. c c The puncta. d e The 
nasal duct. 

The eye has six muscles, which are attached to the 
outer coat, and which give it almost every variety of 
motion. They are among the most curious parts of 
the mechanism of the organ of sight. 

Having thus taken a general view of the eye and its 
appendages, let us next examine the office which the 
different parts perform in the function of vision. It is 
this function that makes us acquainted with light and 
the color of bodies ; it aids us too in forming an idea 
of their size, figure, and distance. But this function 
cannot be exercised without the stimulus of light, a 
subtle fluid, which is constantly emanating from all lu- 



SENSE OF SIGHT. 187 

minoiis bodies. Light is not homogeneous, but is 
composed of seven different colored rays : these rays 
cannot be decomposed. It is by a knowledge of these 
facts, that the variety in the colors of different bodies 
is explained. A white body, for example, reflects all 
the rays ; a black one absorbs all ; and a colored body 
reflects some, and absorbs others. These rays of light 
enter the eye, and form upon a certain part of it the 
image of the body, from which they go off ; when the 
mind perceives this image, vision takes place. When 
the rays strike upon the cornea, the transparent mem- 
brane situated in the front part of the eye, some of 
them are reflected, which gives the eye its lustre and 
brilliancy, and the others pass through it. In thus 
passing from a rarer to a denser medium, that is, from 
the air into the aqueous humor, the rays are refracted ; 
and it may be said, that it is among the uses of the cor- 
nea, the aqueous humor, the crystalline lens, and the 
vitreous humor, to refract the rays, and to concentrate 
them on the retina. 

It is the province of natural philosophy, rather than 
of physiology, to enter minutely into the theory of vis- 
ion. It may, however, be observed, that the laws of 
optics teach us, that the rays of light passing through 
an optical instrument like the eye, must produce an in- 
verted image of the object, from which the rays pro- 
ceed. And this is found to be true, by experiment. 
If the sclerotic coat be removed from the back part of 
the eye of an ox, and the eye be placed in a hole in a 
shutter, by looking through it in this situation, the im- 
ages of the objects seen will be inverted on the retina. 



188 SENSE OF SIGHT. 

The more recent experiments of Magendie have fully 
confirmed the prevailing opinion. 

The principle office, therefore, of the cornea, the 
aqiieoi s humor, the crystalline lens, and the vitreous 
humor, is so to transmit and refract the rays of light, as 
to produce the most perfect image on the retina. But 
there are other parts, whose office is essential to the 
correct performance of this function. The iris, the 
beautiful curtain which is attached at its circumference 
to the circular opening in the coats of the eye, and 
which floats in the aqueous humor, has a slit in its cen- 
tre, through which the rays of light must pass, before 
they can reach the retina. It is furnished with muscu- 
lar fibres, by which it can contract and dilate this slit, 
so as to exclude or admit the light, and regulate the 
quantity of it. This opening in the iris is called the 
pupil, and the variations in its size, according to the 
degree of light to which it is exposed, is a subject of 
common observation. If it be too much contracted, 
or too much dilated, vision will be imperfect. When 
we go from a dark place to one where the light is 
strong, we at first see but imperfectly, because the pu- 
pil had been very much dilated while in the dark, in 
order to allow as many j-ays of light to enter the eye as 
possible, and it cannot instantaneously accommodate 
itself to the great increase of light, so that we do not 
see, from the mere excess of the very cause which pro- 
duces vision. The reverse also is true. Every one's 
experience teaches him, that he sees much better in a 
dark night, after he has been some time in the dark, 
than he did at first, because the pupil has become much 
more dilated, and, of course, admits more light. 



SENSE OF SIGHT. 1'89 

The pupil can be so much dilated as to completely 
destroy vision while the dilatation continues. There 
are various substances that can produce this effect, 
when taken into the stomach or applied upon the skin. 
If the extract of Stramonium, the common Apple Peru, 
be mixed with water, and applied to the skin around 
the eye, the pupil will become extremely dilated, and 
remain so, as long as the Stramonium is kept on. 
Oculists make use of a knowledge of this fact in some 
of their operations, particularly for that of cataract. 
This disease is an opacity in the crystalline lens, or 
its capsule, or both. Couching consists in depressing 
this opake lens, and thus removing the obstruction to 
the rays of light. It is desirable in this operation not 
to injure the iris, which is apt to fall in front of the 
couching needle, if much of the aqueous humor escapes. 
By applying the Stramonium, a short time before the 
operation, the pupil is dilated, or, in other words, the 
iris retracts, so as to be less exposed to injury. It is 
obvious that if this operation be completely successful, 
the eye is not as perfect as if it had never been dis- 
eased, and of course the vision will not be as good, for 
an important part of the crystalline lens has been re- 
moved. It is therefore necessary for those who have 
undergone this operation to wear convex glasses, which 
in some degree compensate for the loss they have sus- 
tained. 

The use of the choroid coat, is to absorb the rays of 
light immediately after they have passed through the 
retina, which it is able to do by means of the black pig- 
ment on its surface. Were it not for this provision, the 



190 SENSE OF SIGHT. 

light would be too intense, and vision would be indis- 
tinct. In those animals who see only by night or very 
imperfectly by day, it has been found that their eyes 
are not furnished with this pigment. The same defi- 
ciency exists in the eyes of albinos; and it is well known 
that they see more distinctly with a small than with a 
large quantity of light. Some white animals with red 
eyes, as white rabbits and white mice, have the same 
defect of vision from the same cause. 

The office of the retina is to receive the impression 
of the rays of light, and it is upon it that the image is 
formed. It may therefore be considered the seat of vi- 
sion; but this does not take place without the action of 
the optic nerve and the brain: the nerve conveys the im- 
pression to the brain, and the brain perceives it. But 
how all this is accomplished we know not. It is the 
result of vital action, which we can neither imitate nor 
understand. 

The retina was formerly supposed to be endowed 
with extreme sensibility; but recent experiments have 
shown, that it is almost insensible to every stimulus but 
that of light. The same is true of the optic nerve. 
Branches of the fifth pair furnish the eye with the ordi- 
nary sensibility, and several pairs of nerves are distrib- 
uted to its muscles. 

There are some circumstances connected with vision 
that may be thought worthy of notice. It is well 
known that a separate image is formed on each eye, 
and that if the eyes be not in the same direction, the 
objects will appear double. This is easily proved by 
pressing one eye, so that the rays of light cannot enter 



SENSE OF SIGHT. 191 

it in the same direction as they do in the other: two 
objects will then be seen, or double vision, as it is called, 
will be produced. This affection arises from a variety 
of causes: it is sometimes the effect of tumors, which 
grow in the socket and press against the eye-ball, and 
sometimes it is the result of an affection of the brain in 
consequence of injury or disease. 

It is well known, however, that the eyes of persons 
who squint are not in the same direction; and it is also 
well known that they see but a single object. Physiolo- 
gists were for a long time puzzled to explain this apparent 
contradiction, and they are indebted to Buffon for a so- 
lution of the difficulty. In squinting, both eyes are not 
directed to the object, and one eye only sees. If the 
individual saw with both, there would necessarily be 
double vision. The cause of squinting is a weakness 
or imperfection of one of the eyes; and if it had the 
same direction as the well eye, it would therefore pro- 
duce imperfect vision. To avoid this, it is uncon- 
sciously turned to one side, usually towards the nose ; 
and this, before long, gives the eye permanently that 
direction. 

When the weakness of the eye is not very great, and 
the individual who is thus affected is young, the diffi- 
culty can sometiaies be removed. If the well eye be 
covered, the other sees, and takes its natural direction, 
and it may not unfrequently be kept so, by compelling 
the individual to use only the weak eye for a length of 
time. 

It is not rare to meet with individuals whose eyes 
are so singularly constructed, that they see best at night, 



192 SENSE OF SIGHT. 

or in a very feeble light. They are called nyctalopes; 
and this peculiarity is owing to a deficiency of the black 
pigment of the choroid coat. It has been found that 
this pigment is more abundant and of a darker color in 
the eyes of those persons who reside in countries in 
which they are exposed to a strong light, than in those 
who reside in countries where the sun is not so power- 
ful. The pigment is very dark in the eyes of birds 
that are much exposed to the rays of the sun, while it 
is entirely wanting in the owl. 

By attending to the structure of the different parts of 
the eye, and considering the manner in which the image 
of luminous bodies is formed on the retina, it is not 
difficult to explain the cause of that very common de- 
fect known by the name of short or near-sighledness, 
The individuals, who labor under it, cannot see objects 
distinctly, unless they are brought near the eye. The 
reason is, that there is too great a degree of convexity 
in the cornea, or the crystalline lens, or the vitreous 
humor, or all of them, and tl.e rays of hght are too 
soon brought to a focus; in fact, before they reach the 
retina. This defect is in a great measure obviated by 
the use of concave glasses. 

Long-sightedness is of course an opposite defect, and 
it is one v.hich most people experience after middle life. 
The different parts of the eye are not sufficiently convex, 
the rays are not brought soon enough to a focus, and 
an indistinct image is the consequence; convex glasses, 
it is well known, are used with great advantage in these 
cases. 

The sense of sight contributes more to the enjoyment 
and happiness of man than any of the other senses. It 



S£NSE OF SIGHT. 193 

is indeed a subject of astonishment, that an instrument 
so curiously constructed as the eye, many parts of 
which are so delicately formed, and requiring as it 
does for the performance of its office the co-operation 
of so many distinct and dissimilar portions, should be 
able for so long a time to perform its functions. It is 
not a matter of surprise that so many are deprived of 
sight: on the contrary, it is wonderful that so many con- 
tinue in the enjoyment of it, when we consider the 
almost numberless accidents to which this organ is ex- 
posed. 

17 



(194) 



CHAPTER XIV. 



OF THE VOICE. 



The voice is a sound produced in the wind-pipe by 
the air in its passage to and from the lungs It is, how- 
ever, most usually produced in the act of expiration. 
Though it is intimately connected with respiration, this 
alone cannot produce it. The functions of the lungs 
may be perfect, and yet the animal may be destitute of 
voice. Its organ is the larynx, and of the structure of 
this we shall now attempt to give some description. 

The larynx forms the top of the wind-pipe, and is 
consequently situated in the upper part of the neck. It 
is attached above to the bone of the tongue, and behind 
it is connected with the oesophagus, the passage to the 
stomach. It is the larynx which makes that remarkable 
prominence that is seen in the neck, particularly of 
men, and which is called Adam's apple, pomum Adami. 
The larynx is composed of four cartilages loosely con- 
nected by membranes, and a fibro-cartilage. The car- 
tilages are the thyroid, the cricoid, and the two ary- 
tenoid, and the fibro-cartilage is called the epiglottis. 
The thyroid or scutiform cartilage, so called from its 
resemblance to a shield, constitutes the front and lateral 



THE VOICE. 195 

part of the larynx. It is larger than the other cartilages, 
broader in front than behind, and composed of two 
parts, with flat surfaces on the side, which unite at an 
acute angle in the front of the neck, forming the promi- 
nence spoken of above. Behind, it has two projections 
at its upper part, which are connected with the bone of 
the tongue, and which are called its upper horns or 
cornua. It has two others below, which are united by 
ligaments with the cricoid cartilage, and which are 
called the lower horns, or cornua. 

The cricoid cartilage takes its name from its resem- 
blance to a ring. It is situated below the thyroid car- 
tilage: it is narrow in front, broader at the sides, and 
still broader behind, where it is connected with the thy- 
roid cartilage. It is not in contact with this cartilage 
in front; the space is occupied by the lining membrane 
of the larynx, and covered by the common integuments. 
It is in this space, that an opening is frequently made 
for the removal of foreign bodies, that accidentally get 
into the wind-pipe. 

The cricoid cartilage rises up considerably behind on 
each side, and on each of these eminences there is a 
smooth surface, on w^hich are situated the arytenoid 
cartilages. They are placed at the back part of the 
larynx, and are much smaller than the cricoid cartilage. 
When the larynx is examined from behind in the dead 
body, before any of the parts are removed, the arytenoid 
cartilages form a single eminence, with a concave sur- 
face above, and resemble in appearance the mouth of 
an ewer, from which they take their name. They are 
loosely attached to the cricoid cartilage, and hav^e mus- 
cles which move them in a lateral direction, and it is 



196 THE VOICE. 

by their motions that the opening in the larynx, called 
the glottis, is enlarged and contracted. The arytenoid 
cartilages are connected with the thyroid by means of 
four fibrous ligaments, the two lower of which are 
called chordae vocales, or vocal chords, as they perform 
an essential part in the production of the voice. They 
are about half an inch in length, and arise from the an- 
terior prominence of the arytenoid cartilages, and pass 
horizontally forwards and inwards, to the angle formed 
by the junction of the two sides of the thyroid cartilage, 
and there meet together. The space between these two 
ligaments, is the glottis, or rima glottidis, or chink of the 
glottis; for it is called by all these names in anatomical 
descriptions. 

At a short distance above these vocal chords are two 
others, running nearly in the same direction; and the 
space between them and the lower chords is called the 
ventricle of the larynx. 

The epiglottis is situated between the larynx and the 
root of the tongue. Its shape has been said to resem- 
ble that of a myrtle leaf. In the ordinary state its 
position is perpendicular, but in the act of swallowing 
the tongue goes back, the epiglottis which is attached 
to it is thus brought over the glottis, whence its name, 
and the food is in this way prevented from passing into 
the wind-pipe. As soon as swallowing is effected, the 
tongue ceases to press upon the epiglottis, its elasticity 
restores it to its former position, which it is important 
for it to preserve, except in the act of swallowing, in 
order to allow a free passage for the air to the lungs. 

The resources of nature are strikingly displayed, 
when, as sometimes happens, the epiglottis is destroyed 



THE VOICE. 197 

by disease. It, no doubt, under ordinary circumstances, 
prevents the entrance of food into the wind-pipe in the 
act of swallowing; but those patients who have lost the 
epiglottis have still the power of swallowing, though in 
an imperfect degree. Magendie, who removed this 
little organ in some experiments on animals, found that 
they could swallow, and that the food was prevented 
from entering the wind-pipe by the contraction of the 
vocal ligaments, w^hlch closed the glottis. 

The larynx is supplied by four nerves, all furnished 
by the par vagum, or eighth pair; two are called the su- 
perior laryngeal, and are sent off in the neck, and the 
other two are called inferior laryngeal, or more com- 
monly the recurrent nerves, because the par vagum 
does not give them off, till it has passed the larynx and 
entered the chest. 

In front and somewhat below the larynx, is situated 
a body, called the thyroid gland, the use of which is 
not known. It consists of two lobes, one on each 
side, which are united together in the middle. It is 
very largely supphed with blood, having four arteries, 
two from the external carotid, and two from the sub- 
clavian. There is no evidence that it performs the 
functions of a gland, as no excretory duct has ever 
been detected going off from it. It is of a very varia- 
ble size in different individuals: it is usually larger in 
fem les than in males, and it is the seat of that singular 
and anomalous disease, so prevalent in some countries, 
which is known by the name of goitre. 

The larynx is undoubtedly the organ of voice. If 
an opening be made in the wind-pipe, the voice is lost : 
17* 



198 



THE VOICE. 



if this opening be closed by the hand, or any thing that 
shuts it completely, the voice is restored. We see 




The larynx and trachea 3oen in front, in outline. 
The thyroid ghuid is shaded. Ji 'J'he thyroid 
cartilage. B 'i'he cricoid cartilage. C The 
trachea. D D 'I'he thyroid gland sealed bflow 
the larynx, and embracing tlio upper part of Uio 
trachea. 



this frequently in persons who attempt to commit sui- 
cide, but who only wound the wind-pipe, or the larynx, 
without cutting any important blood-vessel. 



THE VOICE. 199 

It is also proved by experiment that the epiglottis is 
not concei ned in the production of the voice. An in- 
cision was made in a living animal between the os 
hyoides, the bone of the tongue, and the thyroid cartil- 
age, and through this opening the epiglottis was drawn 
out: the animal retained his voice. It has also been 
ascertained, that the voice is not lost when the epiglot- 
tis is confined or even cut away. Its office seems to 
be only to guard the wind-pipe against the introduction 
of foreign substances into it whilst swallowing. 

The voice, under ordinary circumstances, is produced 
during the act of expiration, that is, as the air is passing 
out of the lungs; but this alone is not sufficient for its 
production; if it were, breathing would be always ac- 
companied by a vocal sound. But this is not the case, 
for the voice is under the control of the will, and res- 
piration is in great me sure involuntary. We can, to 
be sure stop our breathing for a time by our volition; 
but we know that respiration goes on as well when we 
are asleep, as when we are awake, and when of course 
there is no effort of the will on our part. 

Experiments have also satisfactorily shown, that 
nearly all the larynx, except the chorda? vocales, could 
be cut away, without destroying the voice. These 
chords, it will be recollected, passed from the arytenoid 
cartilages to the thyroid cartilage, and are what are 
sometimes called the lower vocal chords. It is obvious 
therefore, that these chords perform an important, if not 
the principal part in the production of the voice. The 
precise manner in which this is accomplished is not 
known, but the explanation given by Magendie is 



200 



THE VOICE. 



perhaps more satisfactory than any other. The air, in 
passing from the lungs in expiration, is forced out of 
small cavities, as the air-cells and the minute branches 
of the wind-pipe, into a large canal; it is thence sent 




A section of the larynx and paitofthe trachea. 
Ji The thyroHl caitihige. B The cricoid 
cartilage. C 'l"he aryii-noid cartilage. JD 
The cartilaginous rings of the trachea. F. 
The superior tliyio-arvtenoid ligament, ex- 
tending from the iliyroid to the arytenoid 
cartilage. F Tim lower thyroarytenoid lig- 
ament or vocal chord. Dotween these lig- 
anients is the ventricle of the larynx. 

through a narrow passage, on each side of which is a 
vibratory chord, and it is by the action of the air on these 
chords that the sonorous undulations are produced 
•which are called voice. 



THE VOICE. 201 

The larynx has been examined in a living animal, 
and at the moment he uttered his cries, it was seen that 
the upper chords were unaffected, while the lower ones 
distinctly vibrated. 

It has been objected, however, to this theory of the 
voice, that when air is forced from the trachea through 
the larynx m a dead animal, the voice of the animal is 
not produced. It has to be sure been maintained by 
some writers, that it can be produced in this way; but 
this statement is not credited at the present time. In 
fact, numerous experiments have been made on the 
subject, which show conclusively that no sound follows, 
unless the arytenoid cartilages are brought in contact 
with each other. But, admitting that the voice does 
result from the vibration of the vocal chords, it does not 
follow that it could be produced when air is forced 
through the trachea and larynx of a dead animal. The 
production of the voice is not a mechanical process, 
which can be easily imitated, but it is a vital function, 
executed by hving agents, under the direction of the 
laws of vitahty, and the result of a voluntary action. 
Even in the living animal, the voice is lost if certain 
muscles be paralyzed. If both the recurrent nerves be 
divided, this effect follows. This fact shows conclu- 
sively that the vocal sound depends upon the vocal 
chords; for these cannot vibrate without the contraction 
of the thyro-arytenoid muscles, and to these muscles 
the recurrent nerves are sent. It is the office of these 
muscles, which go from the thyroid to the arytenoid 
cartilages, to move these latter cartilages, and, in so 
doing, they put in a state of tension, the lower vocal 



202 THE VOICE. 

chords, which are also attached to these cartilages. 
Thus it appears that the voice is lost, when its organ is 
apparently perfect, and the air passes through it as 
usual, merely because two small nervous chords at a 
distance are divided; and yet it is a matter of astonish- 
ment to some, that they cannot produce the vocal sound 
by forcing air through the trachea and larynx of a dead 
animal. 

There can then be no doubt as to the nerves of 
voice; they are the recurrent nerves, which are given 
off by the par vagum in the chest, and are distributed 
to those muscles, which move the arytenoid cartilages 
in such a way as to produce a tension of the vocal 
chords. It seems singular that there should have been 
any question on this subject in modern times; for Galen, 
who lived seventeen hundred years ago, divided the re- 
current nerves of a pig, and thus deprived him of voice. 
The same result has been obtained more recently in 
experiments made on different animals. Magendie found 
that if both the recurrents were cut, the voice was im- 
mediately lost, and if but one were divided it was par- 
tially destroyed. 

It has been a subject of controversy among physiolo- 
gists, whether the larynx was a wind or a stringed in- 
strument. The question is still unsettled, but there 
seems to be no reason why it may not partake in some 
measure of the character of both. 

There are two kinds of voice, the natural and the 
acquired. The former is possessed by the inferior 
animals as well as man; the latter is enjoyed to any ex- 
tent by man alone. The natural voice consists in mere 



THE VOICE. 203 

cries, without any articulate sound. It is possessed by 
the infant as soon as it is born, as well as by idiots, and 
those who are dumb from their birth. It is dependent 
on organization only, and not at all upon the intellect or 
the sense of hearing. The larynx is the exclusive 
organ of this voice. 

The acquired voice is the result of imitation, and 
enables us to give utterance to articulate sounds or 
words. The child soon learns to imitate, the sounds 
which he hears; but those who are born with great im- 
perfection in the organ of hearing, never learn to speak; 
they are dumb because they are deaf, and not because 
there is any defect in the organs of speech. The faculty 
of speech cannot be acquired if there be no sense of 
hearing, or if the intellectual faculties are wanting; hence 
we see that those who are born deaf are dumb, and that 
many idiots are also dumb. The want of intellect is no 
doubt the reason why infants do not speak at an earlier 
age, for they have the organs of voice in perfection, as 
W'Cll as the sense of hearing. There are some animals, 
as the orang-outang, who have the organs of speech 
as well developed as they are in man, but who are 
never able to acquire the faculty of speech. The reason 
which has been assigned for this by Dr. Gall, seems to 
be a satisfactory one, and that is, that it is owing to 
their want of intellectual faculties. 

In the acquired voice, some other organs besides the 

larynx are brought into exercise, as the tongue and the 

.lips ; but neither of them are essential to the faculty of 

speech. There are a few letters, called labials, which 

cannot be well sounded without the lips ; but many 



204 THE VOICE. 

cases are on record, which prove that articulation re- 
mains perfect after the tongue has been removed. I 
can also bear witness to this fact. 

The acquired voice or speech, and the construction 
of language, are among the strongest evidences of the 
superiority of man's intellect over that of other animals, 
and enlarge, perhaps more than anything else, his ca- 
pacity for enjoyment. 

The acquired voice is susceptible of a great degree 
of cultivation. By practice, many persons learn to 
imitate the voices of other individuals, and even of oth- 
er animals. Ventriloquism, as it is called, is perhaps 
among the most curious modifications of the voice. 
Persons who practise this art, endeavor to inspire the 
belief, that in doing it, they do not use the ordinary vo- 
cal organs. But ihis is not the case ; the effect is pro- 
duced chiefly by the power they possess of modifying 
the voice according to circumstances. 

Ventriloquism consists in making the voice of the per- 
former appear to proceed from other persons in various 
places, either in or out of the room. To do this, he first 
directs the attention of his audience to the place where 
the person, who is about to speak, is supposed to be : he 
next produces such a modification in his voice, as would 
make it sound like the voice of a person at the distance 
at which the speaker is supposed to be ; and he also 
avoids the use of those words which have labials, and 
which would of course require the use of his lips. Let 
us suppose, for example, that he has stated that the 
person, with whom he is about to converse, is in the 
cellar : he addresses him in his usual tone of voice; but 



THE VOICE. 206 

when he comes to reply for the man in the cellar, he 
changes his voice, and gives it such a sound as it would 
have, if it had come from one speaking there, and at 
the same time avoids the use of the labials. This can 
only be acquired by long practice, and by those who 
have a strong faculty of imitation. Other explanations, 
and some of them partaking of the marvellous, have 
been given of this talent : the one just stated, is that of 
Magendie, and seems at least to be highly probable. 

18 



(206) 



CHAPTER XV. 



OF LOCOMOTION AND ITS ORGANS 

It Is by the power of locomotion, that we are able 
to change our place. This power, perhaps, connects 
man more intimately with the external world, than any- 
thing else. It of course enlarges his sphere of action, 
and increases his means of acquiring knowledge. It 
distinguishes him from vegetables; for though many 
plants, particularly some of those of the sea, are not at- 
tached to the spot where they originated, they do not pos- 
sess, in the proper sense of the term, the power of lo- 
comotion. This implies volition, and may be said to 
be the result of a voluntary contraction of the muscles. 

In those functions which relate to the nourishment 
and growth of the individual, the animal is passive ; di- 
gestion, absorption, the circulation of the blood, and 
secretion, go on without any act of the will ; in fact, 
they are not under its control, they cannot be stopped, 
if we would. They are, therefore, organic functions, 
connected with the mere organization of the body, and 
which we possess in common with vegetables. 

But the functions of the brain and nervous system, 
those of the organs of the external senses, and that of 



LOCOMOTION AND ITS ORGANS. 207 

the voice, as well as those of the agents of locomotion, 
are strictly animal functions ; for tbey are possessed by- 
animals alone, and vegetables do not partake of them 
in the least degree. 

The bones and muscles are the agents of local mo- 
tion; but they can do nothing, unless they are supplied 
whh the nerves of voluntary motion, and these are un- 
der the influence of the will. But in addition to the 
bones and muscles, there are another set of organs that 
are intimately connected with them: these are the liga- 
ments. The ligaments are strong fibrous bands, which 
connect the bones with each other in many parts of the 
body. They allow the joint, where it is required, 
great freedom of motion, as we see in the knee and 
shoulder joints. They are white membranes, of great 
strength, and endowed, in health, with but a very slight 
degree of sensibility. 

In considering the means by which locomotion is ac- 
complished, the bones, muscles, and ligaments may be 
regarded as the mechanical agents, and the brain and 
the nerves as the vital ones. The nerves convey the 
stimulus of volition to the muscles; they in consequence 
contract, the bones serve as levers, and, being connect- 
ed with each other by ligaments, form joints or hinges, 
and the whole machine is thus put in motion under the 
guidance of the will. This power of contraction, or 
contractility, as it is called, is possessed by the muscles 
alone, and is one of the most remarkable properties of 
life. It performs an essential part in many of the func- 
tions of the body. It is by the muscular fibres in the 
iris, that it is able to contract, when exposed to too 



208 LOCOMOTION AND ITS ORGANS. 

much light; and it is the muscular power of the heart, 
which enables it to throw the blood with such force 
through the arterial system. 

The muscles are of various forms in different parts 
of the body. They are composed of numerous fibres, 
which are connected together by cellular membrane. 
They constitute a great part of the bulk of the body. 
It has been a subject of controversy among physiolo- 
gists, whether muscular contractility was derived from 
nervous influence, or whether it was a property inherent 
in the muscles. The latter opinion was maintained 
with great ability by the celebrated Haller; but the pre- 
vailing sentiment is now in favor of the other ; though 
some recent experiments of Dr. Philip, show that the 
question is not yet considered as absolutely settled. 

When a muscle contracts, its fibres shorten, and be- 
come harder, as may be easily ascertained by placing 
one hand on the inside of the arm, and then bending 
the elbow. The muscle on the inside will be felt to 
contract and to become exceedingly hard. The pow- 
er with which a voluntary muscle contracts, is very va- 
rious : it depends, of course, upon the will. Under 
some circumstances, it is much greater than under oth- 
ers : when the individual is insane, or under the influ- 
ence of strong and exciting passion, as anger, his mus- 
cles seem to be endowed with preternatural power. 
The knee-pan is frequently fractured by muscular con- 
traction alone. The fact that we are able to raise great 
weights, is another evidence of the extent of this pow- 
er ; for this must be effected by the contraction of the 
muscles. Some of the inferior animals possess this 



LOCOMOTION AND ITS ORGANS. 209 

power in a higher degree than man. Some insects, it 
is said, are able to carry a piece of lead equal to their 
bodies in size. 

The length of time in which a muscle remains con- 
tracted, is various. The duration of the contraction 
of the voluntary muscles, is in some degree under the 
control of the will ; but it cannot be continued indefi- 
nitely ; relaxation must follow. The duration of the 
contraction is also in some measure in an inverse pro- 
portion to its force : if a muscle has contracted with 
great violence, relaxation will take place sooner than 
when the contraction has been less powerful. The 
muscles, which produce the various motions of the 
body, are so arranged, that while some contract, oth- 
ers are relaxed ; and if this were not so, it must be ob- 
vious, that continued locomotion could not be effected. 

The velocity of the muscular contractions is de- 
pendent on the will. Though many of the voluntary 
muscles in man contract with great rapidity, those of 
some other animals far surpass them in this respect. A 
race-horse, it is said, has run a mile in a minute, though 
it is not pretended that he could continue at this rate, 
even for a second minute. But rapid as this is, it is 
far inferior in velocity to the flight of birds and insects. 

Muscular contractility is the property which produces 
all the motions of the animal machine. It generates 
power, and therefore differs essentially from a-ny known 
mechanical property. In the best contrived machinery, 
no power is really generated ; but the effect produced, 
is the result of the application of a pre-existing power. 
Contractility, on the other hand, produces power, by 
18* 



^10 LOCOMOTION AND ITS ORGANS. 

the mere effort of the will; and though the animal mo- 
tions are frequently made on mechanical principles, 
they are not always so. The muscles are in some in- 
stances attached to the bones which they are to move, 
not in the w^ay best adapted, according to the laws of 
mechanics, to gain power. It is obvious that all the 
movements of the animal machine are directed by some- 
thing more than mechanical principles, and these are 
the laws of vitality. 

The various attitudes and motions of the body are the 
result of the contractile power of the muscles. We 
cannot be in that common attitude of standing on both 
feet, without putting this power into requisition. The 
body is prevented from falling, by the effoi*t of the 
muscles alone. The base of support is small, being 
only that space on which the feet rest, and that which 
is between the feet. The larger the base, the more 
secure will be the position ; so that those who have 
small feet, do not stand so firmly as those with larger 
ones, and those who have lost a part of their feet, stand 
still less securely than either. The smaller the base, 
the greater must be the muscular effort to preserve the 
attitude ; hence the difficulty of standing on our toes, 
or of walking on a rope. 

The bony structure, which constitutes the solid part 
of the frame, is composed of a variety of distinct pieces. 
These are connected together by ligaments and mus- 
cles, which allow of a greater or less degree of motion. 
The muscles that pass from the spine to the head, pre- 
vent it from falling forward, as it would do were it not 
for them. This we see daily. Persons who get to 



Locomotion and its organs. 211 

slee}3, while sitting or standing up, do not keep the 
head erect : it falls directly on the breast. The same 
thing occurs when a person is suddenly deprived of his 
consciousness and power of volition, as by palsy or 
apoplexy ; the head invariably falls forward. 

The spine is also supported by the power of the 
muscles ; the sanae is true of the thighs, the legs, and 
the Teet. Habit and instinct teach us which position 
of the feet renders us most secure in the erect attitude. 
That has been said from observation to be so, in which 
the feet are placed parallel to each other, with a space 
between them equal to the length of one of them. If 
the feet are more separated, we become more secure 
in the lateral direction, but more inclined to fall for- 
wards and backwards. 

Walking consists in a succession of steps. Let us 
suppose a person to be standing in the attitude spoken 
of above ; that is, with his feet parallel to each other, 
and that he wishes to go forward. To accomplish this, 
he first raises the thigh, by the contraction of the pow- 
erful muscles which go from the body, and which are 
inserted into it. By thus bending the thigh, the leg is 
carried forward ; the foot is then brought to the ground, 
which the heel first touches. The body is then par- 
tially rotated on the head of the thigh bone, which is 
fixed, of the foot that has just been advanced ; the oth- 
er thigh is raised in the same way as the first was, at 
the same time that the body is rotated, so that the leg 
is carried forward, and the foot can be put down at the 
side of the other, or in advance of it. This completes 
a step, and it is obviously the result of muscular power, 
under the guidance of the will. 



212 LOCOMOTION AND ITS ORGANS. 

• In the act of leaping^ the body is raised from the 
ground, and, for a very short time, is suspended in the 
air. When we wish to leap directly forward, we bend 
the head upon the body, the body on the thighs, these 
on the legs, and the legs on the feet. The feet too do 
not stand firmly on the ground, as when we are about 
to walk ; the heel only slightly touching it, or not touch- 
ing it at all. All the muscles, in fact, that are concern- 
ed in the act, are in a state of flexion. They are then 
suddenly and simultaneously contracted with great force; 
by this contraction, the feet are not only raised from 
the ground, but, as was explained when speaking of 
w'alking, are carried forward. When the body is thus 
thrown forward, it is governed by the laws which gov- 
ern all projectile bodies; it is carried to a greater or 
less distance, in proportion to the power with which it 
is projected, and it is brought back to the earth by the 
law of gravitation. The muscles which act with the 
greatest power in leaping, as they must raise the whole 
body, are those of the leg, that large mass of flesh 
known as the calf of the leg. They are admirably fit- 
ted for this, both from their size, and the manner in 
which they are attached to the heel. The body is not 
carried forward by any direct impulse ; on the contra- 
ry, the retraction of the head and the spine would have 
a tendency rather to throw it backwards ; but the rota- 
tion of the thigh bones has an opposite effect : it is suf- 
ficient not merely to counteract the powers that are ex- 
erted in a different direction, but also to carry the body 
forwards ; it is the action of the muscles of the leg that 
carry it upwards. The length of the leap must depend, 
if other things be equal, on the length of the thigh bones. 



LOCOMOTION AND ITS ORGANS. 213 

The arms are of some use in the act of leaping. 
They are brought to the side of the body, when all the 
joints are in a state of flexion; but the instant the feet 
are raised from the ground, they are thrown out. They 
thus resist the muscles which have a tendency to raise 
them up, and thus add the power of these muscles to 
those which are exerted to raise the body upwards. 
The ancients understood this, and it was customary for 
the leapers among them to hold in their hands, weights 
which were called halteres. We see in our own times 
that persons, who are about to leap, take in their hands 
some heavy substance, as bricks or stones. 

Hopping is merely leaping on one foot. The body 
cannot of course be thrown so far in this way as in 
leaping, as the muscles of one leg only are exerted. 

Running consists in a succession of short leaps, ex- 
ecuted by each leg alternately. It differs from walking 
in two respects: 1st, that the body is carried forward 
as each leg is advanced; 2d, that before the foot that is 
advanced has reached the ground, the other is raised 
from it. A momentum is in this way acquired at each 
additional leap, which has the effect of preventing us, 
when we are running rapidly, from stopping in an instant, 
as we can do when we are walking. 

There are numerous other attitudes and motions of 
the body that might be considered under this head; but 
these are sufficient for our purpose, which was to show 
that they were all produced by the action of the volun- 
tary muscles. 



(214) 



CHAPTER XVI. 

OF THE DECAY AND DISSOLUTION OF THE BODY. 

All organized beings have a limited period of ex- 
istence. This is longer or shorter, as the vegetable or 
animal arrives later or earlier at its maturity. The oak, 
which is years in attaining its growth, is for ages the 
tenant of the forest; while those plants and animals, 
which are brought to their full perfection in a few days 
or weeks, decay and die in a period equally short. 

The organs of the human body do not arrive at ma- 
turity so soon as those of most other animals, and of 
course the duration of man's existence is longer than 
that of most of the inferior orders of the animal kingdom. 
The elephant and some other animals, however, surpass 
him in this respect. 

The various functions of the human body bear a very 
different relation to each other at the different periods 
of life. Before the body has attained its maturity, all 
the nutritive functions are in great activity; digestion, 
absorption, and nutrition are rapidly performed. The 
brain and nervous system, which impart vigor to them 
all, are more developed in proportion, than in the sub- 
sequent periods of life. When the body has arrived 
at its full growth, an equilibrium seems to be established, 
and for some years the balance is preserved. 



DECAY AND DISSOLUTION OF THE BODY. 215 

Why should not this state of things continue? Why 
should not the body go on and perform its various 
functions for an indefinite period, if not prevented by 
accidental circumstances? We can discover nothing 
in its structure that renders its decay inevitable. To 
us, judging only from what we know of its organiza- 
tion, there seems to be no reason for its decline and 
dissolution, if it be carefully guarded from injury. We 
know that there is in the system, a power of repairing 
many of the injuries which the body receives; if it were 
not so, wounds would never heal, a slight cut would 
produce a fatal bleeding, and the ends of a broken bone 
would never unite- But the moment any of the organs 
are injured, a process is commenced for their restora- 
tion. There seems to be a sort of conservative prin- 
ciple in the system, w4iich in some measure guards it 
from the effects of our imprudence, and repairs the in- 
jury which it may in any way sustain. But notwith- 
standing the apparent perfection of the organization of 
the body, and the efforts which it has the power of 
making to remove the effects of violence, it will decay, 
be the exertions ever so persevering and well-directed, 
that are made to guard it. It cannot be said that its 
early death is to be attributed to civilization ; to the cir- 
cumstance that man does not adhere to the mode of 
living which was intended by his Creator; for it is cer- 
tain, that in a civilized state, man arrives at a much 
greater age, and retains for a longer period all his fac- 
ulties, than in a savage one. Civilization, so far from 
having shortened the term of human existence, has un- 
doubtedly lengthened it; and that state seems to be the 



216 DECAY AND DISSOLUTION OF THE BODY. 

most natural for man, In which he surrounds himself 
with the greatest means of rational enjoyment, by the 
exercise of his intellectual and bodily powers. 

But in this state, the corporeal organs, though they 
be carefully watched, constantly preserved from injury 
and exempted from disease, will gradually but steadily 
decline after adult age. All the organs do not fail at 
the same time; those, which are essential to the mere 
nourishment of the body, are the last to feel the influ- 
ence of age. The nervous system is among the first to 
lose its vigor. The senses of seeing and hearing 
become dull, and perception is blunted. The powers 
of locomotion are impaired. The muscles contract 
feebly, and remain constantly in a state of partial flexion, 
which renders them unable to support the body per- 
fectly, and gives an attitude of stooping. The step 
is less firm and frequently becomes tottering, so as to 
render the aid of a staff necessary. All these effects 
are the result of an enfeebled state of the nervous system. 

As age advances, the circulating system performs its 
functions less perfectly. Bony matter is deposited in 
various parts of it; the valves of the heart and many of 
the arteries become ossified, so as to interfere essen- 
tially with the free circulation of the blood. 

The purpose of respiration is not perfectly effected 
in consequence of the state of the lungs, or the minute 
capillaries which carry the blood into them. At any 
rate, breathing does not produce in this fluid the com- 
plete change which it does at an earlier period. 

The three great systems therefore, the nervous, the 
circulating, and the respiratory, all become affected by 



DECAY AND DISSOLUTION OP THE BODY. 217 

age, but not all at the same time, nor in the same order 
of succession in different individuals. So intimate is 
their connexion, that whichever is affected, the others 
xire sure to suffer, and the functions of all are before 
long equally embarrassed. When the organs are in this 
enfeebled state, it requires but little to stop their action, 
and this is frequently done by accidental causes, which 
at an earlier period would seem trifling. The powers 
of vitality are so feeble in old age, that they are unable 
to make any effort to relieve the system, and what 
would at one time have been a temporary suspension of 
the action of some of the organs, now produces a ces- 
sation of the functions of all, which is death. 
19 



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